Here's some detail on what happened guys. I must say I really appreciate all your help.
The flange arrived and I fitted the lever valve beforehand, so if I dropped it it wouldn't fall through the cylinder:
I drilled the hole, my hands were shaking
. I read and re-read the instructions and followed them to the letter. The only problem I had was that the supplied hole saw was poor. Blunt, teeth wildly off centre, one tooth sticking out at 45 degrees. So I braved my 3 year old hole saw set and it went through no problems.
I was concerned that the copper was a different colour nearer the seem...not sure whether this is due to the extra heat there or because it's thicker, or what. At one point the cylinder looked like it was buckling under the pressure I was exerting, so I backed off and used slightly higher revs.
Thankfully the biscuit stayed on the drill bit. Even if it makes no difference, I'd have been peed off if the copper disc had fallen in.
Flange fitted:
Close up of flange:
Very close-up of flange, note the red dot so you know where the upward bend is pointing inside the cylinder:
Pipe work, not sure on how correct that run of pex to the mixer is:
Pipe work:
Shower side, something else that cropped up was that the original muppet installer also failed to screw the shower mixer mount into the wall. The mixer was hanging off the compression copper and the silicone on the silver discs that cover the pipe holes:
So tomorrow it's off to B&Q to spend more money on some diamond drill bits for the tiles.
Thanks again for all the help.