It's difficult to know exactly what is going on with the roof from your explanation, but I would guess you may have a shared gulley with an adjoining property. A gulley is different from a valley. A valley is on the pitched part of the roof where 2 roof slopes meet. A gulley is a flat area where 2 or more valleys might meet.
Not knowing exactly what's going on, I would recommend that you get a 2nd opinion with a different roof tiler to take a look especially if you are still getting water ingress.
I would also say that if the property is 100 years old and still has the original roof, then I would think it's about time to have a complete re-roof.

Good Evening. When the bathroom was plastered was it directly onto the old ceiling or did you completely rip out and re plasterboard onto the joists? If plaster was applied directly onto the old ceiling then a historic water stain can appear again as a yellow stain but not necessarily be a water leak from the roof. Personally i would try a product called Zinsser Bin which is a super concentrated stain covering paint. Give the effected area 2 coats of this and if it doesn't appear again then problem solved. If it re appears then the roof could possibly need further analysis. I hope this helps.

I think you should have a complete roof inspection done before spending more money this is my advise

Hi There, Without us looking it is hard to say but the roof should not be continuously leaking because the water is gathering? It doesn't cost anything for another opinion we are happy to take a look and give you our expert opinion.
Thanks Colin Clark.

像一切都很难说没有看,just because your a roofer it doesn’t always mean your a good one , infact most roof repairs I’ve done have always been because the last roofer done a bad job , if you do it right then there’s no reason why it shoudent last 50 years plus , and no roof should gather water in any area water should always run itself off in some way and that’s a fact , maybe your problem just hasent been seen properly