Hello Stewart
When I replaced my winches I found that the nuts to the screws were glassed in to a grey resin like material - which was quite brittle and crumbled easily. It would have been possible to simply unscrew the old winch and screw the new ones straight back on as there was a good chance of the nuts being retained.
The deck is about 6mm thick at this point and there was not any sign of backing plates! It is a top class bit of grip moulding!
My headlining is about half a mm thick and I bolted in two small round barton hatches to access below the winch.
(My headlining may be different to yours on Shivra. I cut into the headlining using a pair of dividers to scribe the circle and scratch through... but it was still a struggle to access the nuts as there is not much clearance between the headlining and the deck at this point.)
I used Antal winches, having exactly the same footprint as the original Encks? No10 and replaced with self tailing No16s, a single speed to the port side and a two speed to the starboard, where my Keel is winched to.
I can send photos if that helps, may need a current email from you. . My details are in the handbook
Best Wishes
Lindsey Moscrop P21 Amy No75