I have a problem with the motorised valve. The CH would come on when hot water only was selected. I detatched the actuator and watched the drive move to the correct position. The question is this: is the valve sticky( i have excercised it and it seems to be working) and on its way out? or is the problem more likely to be the actuator not being strong enough? which is the cheaper?
Assuming the motorised valve is separate from the boiler, proceed as follows.
If you can find the same make and model, it is straightforward to replace the actuator. Make careful notes of which wire goes where and remember to isolate the electrics by removing the fuse to the boiler.
If this does not work, because the seals are not working properly, you already have the body to instal.
If you have to buy a different make, identifying what wire goes where can be a bit of a nightmare. If you have any doubts about your electrical competence, call in a professional