In message , Harry Bloomfield
writes
After serious thinking Graeme wrote :

The washer looks to be extremely worn, but how do I get it out of the
slider? More to the point, how do I get a new one in? Once out, is
it just a rubber disc?

A sharp small flat screwdriver, to hook it out. Most of them are a
shallow cup shape - note the correct face to face in to the valve. Any
plumbers merchant will have a replacement, but might be worth buying
the complete valve for what the cost.

Fixed yesterday and the good news is no overflow this morning. It was
usually OK during the day, as water was being used but overnight the
level crept up, and would trickle out of the overflow by morning.
Fingers crossed!
--
Graeme