After serious thinking Graeme wrote :
Our cold water tank has started overflowing, so this morning, up in the loft,
water off, partially drained cold tank, removed end cap from valve, removed
split pin, wiggled the float arm out and removed the slider thingy which has
a rubber washer in the end. 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.