Thanks for the reply, J.
I'm almost loathe to say what I've done, for fear of an immediate responses along the lines of, 'Of course that's obviously a repro,' 'You've thrown your money away', 'What have you done, you fool, you FOOL!'
I'd been hankering after an original Back to the Future one sheet for some time, having bought a shop reprint back in 1985 and after seeing prices gradually creeping up for the stashes on eBay supplied by the likes of Movieposterzone - who I've read had somehow obtained 'lifetime supplies' of the studio issues.
Moviepostercenter are now offering the rolled poster at the lowest 'buy it now price', advertising it with the correct 27x41 dimensions, and via an email exchange I've been assured by the seller that it is an original, obtained back in the day from Universal.
The seller's ratings are excellent - bar that one neutral comment you noted from one Batman poster buyer who doesn't seem clear themself about what they should expect - so with a 15% off deal on eBay I took the plunge.
As far as I can see I've no reason to doubt it'll be legit, but there's always that lump in the throat when throwing down a fair sum with a seller like this for the first time.
Desk