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So it was the pop up overflow plug unit. Where the cable activates a lever which turns a shaft which goes into the waste pipe for the pop up doobry, there was water coming out at the shaft when the bath drained. I was surprised how easy the unit was to replace. In the picture you can see where the water stains start near the rectangular square box which houses the cable and shaft lever thingy. Still brand new drainage in the bath so just the damp joists to dry out.
Ramsaye
There's a seal on mcalpine stuff that we basically tighten by hand and give a nip. You shouldn't need to PTFE it or anything like that.
You could just replace it with a new shallow trap like that, but stripping it to see the cause is the first thing but it also looks like its running up hill from the photo too but that shouldn't cause the leak more running away.