Hello Ed, thank you for you reply.

It's possible that the 3 port valve did not go fully to HW only before it
was repaired?

The spring went west, the motor got tired, and the shoe got gummed up,
I believe that the three way at this time is functioning from a
diversion point of view as it should.

The best by far if you can afford it and the boiler supports it is to go
for a sealed system. See FAQ.

I am considering this as a long term fix - it's a Keston Celsius 25
and I really don't think this boiler is happy unless it's sealed.

You could try putting a higher loop on the vent pipe so that it requires a
bigger pressure difference to start pumping over.

Planning to do this at the weekend - the better half got me the raw
materials, all I need to do is ensure I don't set fire to the loft.
Any 'guesstimate's on whether I should make the loop as tall as the
loft roof, or go for something less

You could route the feed into the vent pipe - this will make refilling the
system harder but definitely fix the problem.

I could put a T-Junction join in the loft between the vent and the
overflow tank - or would the join need to be lower down in the airing
cupboard?

You could try a lower pump speed and provided it still actually heats the
house and HW. It might also cause the boiler to kettle.

Unfortunately the pump is modulated directly by the boiler.

Many thanks,
--
Steve