Drayton Mid Position Valve sticking

[Michelin]

Hi, My mid position valve is sticking. Regardless of thermostat or programmer settings, the heating goes on with the hot water every morning for an hour (been doing that all summer ).
Or, if I put the heating on for an hour it takes about another hour to unstick and go back off.
Can the top of the mid position valve be replaced without draining the whole system and interfering with the pipes?
It's a Drayton motorised valve and I was told by a plumber (who claimed to know all about heating) that my heating system needed to be drained and all the pipes in the airing cupboard cut to accommodate a new valve; yet on the Drayton site it looks like the plastic valve box just fits on after the pipework is done.
Any help would be much appreciated!

 

[ChrisR]

If the head has no button on the side to pop it off, your plumber's right.

If it's been installed tight, you do have to cut a pipe or two sometimes.

 

[Michelin]

It's got WMH on the side (which sticks) and a waggly black button on the bottom. The unit may not be installed the right way up though (the writing is upside down)

 

[Michelin]

Does anyone know if the waggly black piece at the bottom should be used to remove it? It doesn't seem to set either way, it's just loose.