I posted a similar question mate and the resounding answer was to lift and tile underneath.
As in the advice I got from my fellow learned tilers on here, make sure you get 100% coverage underneath.
One
tiler said carry a load of Flexi pipes with you, get them in bulk.
I myself had a job and the customer was happy to get a plumber in to connect the pipe back on the loo. (took 3 days to get the plumber out like lol hope they didnt clog up ha ha.) teh difference being about 6 mm
I am competent at plumbing but the job I had I had gone in to re-tile after a pipefitter mate of the customer had fitted the bathroom anfd tiled the walls and floors. He had laid 6mm chipboard down and laid the floor tiles down with wall adhesive!!! Needless to say they came up.
That was the reason I didnt want to touch the plumbing. The old thought of if anything had went wrong with the plumbing me being the last one to do anything with it would have come back to me.
imm waiting for them to ring me when hs wall tiles come off but thats another story