No problem at all Tom, it's all interesting stuff. In fact, if I felt my car was worth the outlay I'd be forming an orderly queue at your door clutching my £200.
I eventually went on Youtube and watched some Americans do some horrible things to their cars. One was interesting enough though that I tried it, but kinda wished I hadn't. It involved injecting the spray glue through the fabric via a hypodermic needle welded to the can nozzle. I tried it out on the other back corner of the car where there was some hardly noticed sagging which hadn't actually come away at the edges. Even the tiniest split-second burst, and patiently waiting for the solvent to evaporate, resulted in glue-stains penetrating to the face of the fabric and leaving dots of dark staining when I smoothed the fabric back. Maybe I should have waited much longer than the American. It did stick the headliner back though.
What does seem to have worked though.... fingers crossed cos I only did it today and it could all peel off again by tomorrow.... involved sealing the biscuit with a couple of coats of PVA, after I'd lifted off any loose crumbly bits of the ruined foam backing from the biscuit using some Duck tape, then covering all exposed and accessible biscuit with strips of a pretty good double-sided tape called "Ever-Build Mammoth Tape Carpet Fix Cloth Tape", paying special care to fully cover all the edges of the biscuit. With the headlining having lost most of its foam backing it had become very stretchy and this made trying to get it stuck evenly over the entire area the hardest bit, but I managed it reasonably well and although the finished result is no way perfect, it no longer hangs down, has no noticeable wrinkles, and it no longer bothers me anywhere near as much as before.
It's not a job to be proud of, but it's done and it doesn't notice unless you make a point of looking for it. It's still stuck 4 hours later, but lets see how it survives some more hot days said to be with us again.
This must all seem highly comic... even annoyingly so to the A2 snobs... but I can't believe you all have concourse condition cars and, even for some owners, if the occasional bodge like this one buys some time until a proper job can be done, well... why not.
I'll just keep looking out for that low-miler TDI SE to replace this one. Silver, but maybe Cobalt Blue... at a push.