Honeywell BS864-2 22mm possible to replace head without draining?

[NIgel Hawcroft]

I may have a faulty motorised valve head on a Honeywell BS864-2 22mm is it possible just to replace the head or must it be the whole fitment? Do you need to drain the tank to fit a new head on the valve?
Thanks

 

[ShaunCorbs]

Does the head have a dimple on the top / lid ?