Hi all!
I've stood on the shoulders of giants to bring you the saga of my headliner replacement, so credits coming. Needless to say, thanks to EVERYONE who's helped me and I hope this might come in handy to anyone approaching this as a project in the future.
Split up into a few digestible parts, have a gander at my thrilling escapades. You'll laugh, you'll cry, you'll remove your headlining.

PART 1
Right, well it was a mammoth effort to get this done. Seeing as I'm a 'cack-handed oaf' and got no skills with regards to cars or DIY or anything, embarking on the project at all probably wasn't the most sensible thing to do, but I did it anyway. So go me.

Worth pointing out that I ballsed it up. A bit. Only a TINY bit but I've written this guide up anyway, to serve partly as a cautionary tale rather than a step-by-step expert hand-holding, because you definitely should NOT bugger it up like I did. However I was damn close to all-out world domination headliner styleee. Read and understand and pick holes in this entire guide before attempting anything - if you stuff it up too don't blame me (I've suffered enough!

) and I won't be responsible for any damage to you or your car from use or misuse of anything here. We are taking seatbelts off after all, and you may need those to save your life.
I've also taken bits of this guide from elsewhere as I worked from them, and I've tried to reference thoroughly at the end.
FOR GOODNESS' SAKE CHOOSE A WARM AND NON-RAINY WEEKEND TOO!
Ok, so I bought this Calibra, as we are wont to do around these parts:

BEAUTY ain't she? But there's been something wrong since I got her:

Ewwww! And it was even worse at the back! The fault was due to a rotten rubber seal on the base of the aerial, but DURING the procedure I found ample time to address that too using links and info from
one_more_inch's guide. If that's your problem too then all well and good, but if there's anything else leaking it will be worth checking out the sunroof or seals, as you'll definitely get sick of doing this every few months...
Right then, to work. It'll be about a 7-8 hour job OR a 5 hour job one day, then overnight leave the adhesive, then budget another 3 hours next day.
What you'll need is:
-
Granville black silicone sealant (optional if re-sealing the aerial and gasket, just seal any extra bits that need it up in there.)
- Sharp Stanley knife.
- Torx screwdriver set, flat blade and Phillips screwdrivers or a GOOD multi tool.
- Pliers.
- Socket set/plug spanner to (sigh) remove the seatbelts. Yes really.
- Plastic dish/pan scrubbing brush (scouring pad a VERY poor second, wire brush FAR too harsh.)
- Sellotape to keep bits of trim together and organised.
-
Trimfix headliner adhesive. There is also a 3M product (probably the same/better.) 2 cans will be necessary.
- Elephant tape or the ULTRA-brand Duck tape. Regular Duck tape won't cut it for this.
- New headliner material. Choose whatever you'd like really! There are even complete kits for the process on eBay. I chose cream leather-effect vinyl to match my cream leather seats, but they even do fake purple crocodile-skin, so y'know go nuts

Get about 2m by 2m which will leave you LOADS AND LOADS spare as I
think I recall measuring about 150cm by 130cm of material out for a generous fit.
If you have a car interior trim removal set then break the sod out, but if not then no bother. I didn't have one.
Righty ho, to work! Stage 1 requires us to remove the front interior top trim as shown, the pillars, the b*stard seatbelt b*stards (front and back,) loosen the rear top trim and remove the sun visors, the interior light, the handle, the sunroof control and motor cap, the alarm cowls and alarm sensors and not lose any of the screws and bits. (I kept all little plastic bits/screws in the shallow holder behind the handbrake

)
From the front then, unscrew the visors and clips. These are Phillips screws:

Notice the connector for the visor lights. Hold plastic bits and pull apart:

The sunroof control should prize and pull out alright, and the power connector just unplugs:

Alarm cowl unclips revealing this, which unscrews. The little hooks above the seatbelts push up or prize up from the bottom and reveal a screw:

Undo that torx screw and pull the seatbelt down as far as it'll go. Pillar trim should now be loose and you'll see how the front and rear trims nestle in behind that now.
Interior light clips out of the lining and leads pull out of that, also remove the little circular plastic sunroof motor cover to the rear:

Starting to come apart now huh! Ok, now for a meaty bit. The front top trim has one torx screw behind a cover, the centre pillar had one behind that little hook above the seatbelt, and the rear top trim has one towards the back behind the seatbelt (you'll see all these.) Proceed as follows:

Now the bits you'll be undoing which attach the plastic trim to the bodywork look and work vaguely like this:

Some of them WILL break or come out of the plastic when you pull the trim loose, but none of them or their fixings outright broke on me. It takes a bit of pulling though! Start from the centre pillar edge when the torx screw's out, and pull until the headliner's nice and loose, but don't pull it out all the way just yet so it's still supporting that headliner a bit. On the other side you have the passenger grab handle too, and that just unscrews:

Trim around the sunroof just unwraps from the join:

Now for the REAL sod as we're taking the seatbelts off. If you're not 100% confident this won't go wrong and you're not 100% alright with any potential consequences then don't do this; it's not too late to turn back! I've done this however, and have to say it's all completely fine and my seatbelts have been unofficially judged as such by an ex-Vauxhall mechanic since (quite honestly.)
Plug spanner in hand, take a look at the bolt on the seatbelt (if your plug spanner is like mine, ffs use a Phillips screwdriver as a flatblade may slip and SERIOUSLY hurt you:)


Position it and yourself carefully so as not to slip and get hurt, nor go through a window. Turn and undo the bolt.
Nothing happens.
Turn it harder to undo the bolt.
Nothing happens.
The level of force you need to undo the bolt here is HUGELY disconcerting! I mean, I can bench 90kg for sets and curl 20kg dumbbells for fun, and even I (Schwarzenegger lol) found it super super hard going. They will eventually release with an almighty ''crack!!!'' When off, sellotape the little kebab of bolt, washers and belt positioner together so it doesn't all slide off and bounce away. Same now with rear seatbelts.
Last stage and you should really be seeing the headliner looking loose now! If like me you've noticed the headliner is 'caught' at the front in the area atop the windscreen centre, it may be a small amount of glue holding it to the roof there. I had this and prized it off.
REALLY loose now huh?
Ok, undo the last torx screw and loosen any remaining top trim front or back:

Be prepared for it not to fall down at the back, as there are three little clips attached to a strip of metal which hold it up. As and when you start to pull it back and out the boot these come out anyway, but might need guiding:

It needs VERY VERY GENTLY pulling, side to side, out of the open boot.
THIS IS THE PART WHERE I SNAPPED IT! I was also being SUPER MEGA careful so it is easily done. Do remember you're dealing with a very feeble sheet of what is essentially polystyrene made 20-odd years ago. Mine broke along this area where the sunroof gap is (a weak point but also a mercifully flexible one!) and happily I fixed this with 2-3 wraps of Elephant/Ultra Duck tape.
B*llocks:

Okay now, is it out? In one piece or repaired now like mine?
You should be greeted with something like this:

Stay tuned for part 2, starring Tom Cruise and Gary Glitter.