So what have I learned today?
1. The advice on this forum is truly invaluable.
2. It is quite hard work to remove the headlining. It took me just over three hours. TripleBee's comprehensive post is very informative. There are a few gaps for the likes of me who are enthusiastic DIY-ers rather than trained and experienced automotive mechanics. The main problems I had were in relation to removal of the control and light console and cabling issues. I ended up removing everything including the interior mirror, which was quite taxing. I took some pictures and will post them in a few days. I am not sure it was totally necessary to go that far but I couldn't see any alternative to allow the headlining to be removed, due to the cabling.
3. Still no sign of leaking oil in the interior of the van, which is a relief.
4. The mastic seal (pictured earlier) surrounds some wiring and two hydraulic pipes. Bet you all knew that already. I'm now 100% convinced that one of the pipes is damaged and leaking underneath the mastic. I'm also fairly convinced the damage was caused by work done in that area by the VW dealer earlier this year, but I have no faith I can win that argument with the dealer and I'm not going to give them another opportunity to mess the van up.
5. Need to order the cables ready to fit next week. This looks like a worse job than removing the headlining. No doubt plenty more questions will arise.
Questions such as ... does anybody know where to get hold of a syringe suitable for topping up the hydraulic oil?
By the way, the very expensive oil seems to be getting cheaper. They are only charging me £73 for a bottle. That's still ridiculously expensive, but having been primed to expect £120 plus, I somehow feel OK about it being "only" £73.