Unable to find source of leak

Check the seal around the bath, especially if you have an in bath shower, remove the bath panel and use the shower to spray water around the seal, keep an eye under the bath and see if you can see water trickling down the wall. It may not be enough water coming through to have water coming through your ceiling.

Most common sources are the bath seal, spray around the bath whilst checking underneath to see drips etc. Next use blue roll paper or a coloured kitchen paper and wipe around pipe joints, so your basin pipework, up to the where they connect to the taps, same with the bath and the toilet. If nothing there turn on the taps and wipe the waste pipes, trap etc . Keep check the paper if it's wet it will show up on the paper. Sometime out eyes can't see leaks so small, but that paper can pick it up. Especially when it sounds like a gradual leak

these leaks are something of a pain, I have had many to resolve it is just a case of carrying out a methodical process of elimination it could be the bath seal as suggested by a previous poster but I have had any one of the appliances you typically find in a bathroom as the source also from pipe connections, joints in pipes, even central heating pipes under floor, if you can not locate it yourself why not post the detection of the leak as a job. I am sure someone would pick it up BPS.

I am assuming that the pipe you refer to replacing is the overflow pipe & that the loo is directly above the problem area. With reference to your thought that it may be coming from outside, well yes that is a possibility. it is not uncommon to see a WC overflow pipe badly fitted, in that it can be seen to be running uphill as it comes out of the wall. What will then happen in the case of an overflow is water will the trickle back along the pipe to the wall & cause dampness, especially if not well sealed. Your plumber should have investigated for any overflow & dealt with it as a matter of course. If it is the case that the pipe is running uphill, the overflow has been attended to & the problem still occurs, then it could be rainwater collecting & running back along the pipe. Failing that & with no obvious wetness around the toilet, if it is a back to wall design then it is likely that the soil connector seal has failed so will leak every time it is flushed & if it has an internal overflow then it may be a persistent weep as well. My advice is to first check the angle of all pipes from the loo that go through the outside wall. Hope this helps.