Probably started leaking at the flush cone because you've been disturbing things - always happens!

seems to be fitted just with putty/mait.

I’ll have a look at fitting a new flush cone.

Put a smear of LS-X around the flush cone, it was only slightly, possibly, damp when adjusted.
Bought a new Thomas Dudley flush valve, but the cable was too short, so, upon further investigation, this was the button issue - not sure if it was supposed to have a terminal on the end of the wire:


This is testing my patience - not wanting to fix what isn’t broken, I’ve replaced the diaphragm washer on the Siamp fill valve (cleaned the existing one, but it was still sometimes slow to fill). Didn’t touch or change anything else, but now the cistern will sometimes continue to fill until it goes down the flush overflow, even though the float/tray has popped up and there doesn’t appear to be anything flowing via the diaphragm washer. Everything looks fine, the washer is seated correctly, changing the float/rod height doesn’t make any difference. Can water enter the cistern aside from via where the diaphragm washer assembly is?

so I’ve siliconed everything back together.
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