It will be a 760x760 tray with 10mm each side taken up with tiles/adhesive/plaster/plaster board. Good luck with taking the old tray out without damaging the tiles!
You should find that a 760 door will adjust down to the size you need with the difference taken up in the movement within the profiles...
check the small print before you order though.
One of these taps broke on me a few years ago. I wasn't brutal with it... it just sheared off.
I use my T bar with ONE HAND to close the water off, clockwise as you look into the chamber, so I don't over do it.
I use strapping band, cut 5 holes long, fixed with a 2" screw in the middle hole and bent over the pipes (10mm copper covered in duct tape). Never had a problem other than customers drilling through the pipes for curtains poles or tie-backs.
Sometimes it can be very easy plumbing wise but much harder and expensive electrically speaking. Some MCB blah blah electric stuff are wholly unsuited to a modern electric shower and need upgrading to match current electrical standards. I am a plumber and know several electricians... and vice...
Air was trapped in the system and the pump [circulator] wasn't able to cool the boiler as quickly as the burner warmed it up... hence it got too hot and the limit stat kicked in to protect the boiler. All sorted now glad you are sorted.