With the the matrix replaced, I thought it best to conclude this thread with my findings as it may help someone else in the future.
The following applies to my car, which is a MK2 Xantia.
I think someone else has stated this. The steering wheel does not need to come off. The column is best lowered for easier access but that's all.
The scuttle was the biggest (pleasant) surprise in that it can peeled away with no risk to the 'screen. There's a ridge on the scuttle underside that probably confuses the issue if people mistake this for the seal and try cutting into it. What we found worked well was to gently seperate the scuttle from the 'screen at the very corner where it becomes a slim point. Once you can get a small lever/wedge in there, the seal/bonding should start to tear away from itself. Once I could get my fingertips under the scuttle, the rest was plain sailing.
Jim was quite right, the dash bulbs are capless types, I just didn't want to pull that hard for fear of breaking the glass envelope and therefore assumed they wouldn't pull out but the parts shop assistant showed me a neat trick to remove them. Wrap a piece of masking tape around the bulb, changes the surface from slippery to grippy
As my eyesight is degrading, I got out my microscope to check which bulbs had failed or look burnt enough to be a concern and I'm glad I did! I've found two (so far) that have a broken filament that still makes contact - ie they illuminate despite being broken.
As for drying out the carpets, they don't. It appears the coolant chemicals leave a sticky but wet residue that will resist drying. I've got a towel I used to mop up the spill and despite being wrung out and left in the hot sun for at least three days, it's still very damp.