About a couple of years ago we had a new bathroom and second toilet and
wash hand basin fitted by a plumber.

The upstairs cistern started to leak and I found out that it was a
cracked pipe. The pipe that comes out of the cistern is plastic and the
plumber got the angle of the copper pipe wrong, so the plastic pipe cracked.

Now, the downstairs loo has sprung a leak, but I can't yet make out why.
The plastic pipe flange looks parallel to the tank, but it leaks at the
back. I've tried Boss and adding and changing the seals, but it still leaks.
Before I get out the silicone, is there anything else I can try?

Tomorrow, I am going to get some feeler gauges out and see what the gap
looks like, because I can't see it at the back.

Dave