Any more suggestions? I see a new motorised valve is around £31.00 so I
want to be sure it's that, and if it is, can I fit one myself?

It sounds very much like the valve.

There are 2 main parts to the valve, the water bit with the valve parts and
the "head" which contains the motor and switches. It is normally the head
that goes. It is easy to replace just that part without having to drain down
or dismantle any water carrying parts. It is not much harder than wiring a
plug.

A quick test to determine whether it is just the head is to remove it and
try to rotate the valve "paddle" manually. If it is very stiff, then the
valve might be shot. If it moves easily, then it is probably the head,
although it isn't a absolutely fool proof test.

Heads often don't cost much less than the complete valve, so you might
consider buying the whole thing, so that if it turns out that the valve
itself is knackered, you've already bought the bit and can reunite the new
head and valve.

Christian.