Well, between the two of your kind replies, I started to think my last comment was perhaps more germane than I'd really intended. The warming up issue meaning the glass had to be on top to get the reflectivity.
Late last night, I had set the board to as near to centre as I could gestimate, and it was no different. So, just now, I put the cover on and the darn thing goes!
A bit too soon to go Yaaaaaaaaaaaaaaaaaaaaaay! perhaps, because I've had the ribbon out twice to check the continuities. They don't like being messed with, so I hope it will survive.
Anyway, thanks again and keeping fingers crossed.