The tank will either be your cold water header tank or the flow and expansion tank for the heating system and the valve is a type 1.
Overflow pipe from tank in attic.
With this type of valve the brass stem is bent downwards carefully to adjust the water level.
If not then u will still have a expansion tank in the attic with an overflow.
If its the toilet it will be directly connected to the white overflow pipe so should be easy to trace.
If the water in the tank or cistern fills up to a higher level than it should the excess water flows down the overflow pipe and runs outside rather than leaking into the room.
I think you might be right about the tank on the landing.
How to diagnose and repair a dripping overflow.
Unless your pressurised hw system is in the attic too then it could be a blowoff safety valve no idea about heating system the heating is run from a stanley oil fired cooker don t think it s pressurised though.
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It could be that tank or it could be a toilet overflowing.
The tank in the attic has been trickling out of the overflow for a few weeks now.
An overflow pipe leads from a water storage tank or cistern inside your home to the outside.
Looking for some advice.
They re a vital part of your home s plumbing system.
Have already changed the valve and float in the attic.
Set it very low but water is above the float.
Tested it fine know it shuts off the feed that way.