Oil Worcester HeatSlave 20/25 - hot water temp drops

[burningoil]

Apologies, I posted this last week but think I must have deleted it in error. Basically we have 15 year old oil combi, serviced 8 months ago bu oftec reg. eng. (plus new expansion vessel fitted). Radiators heat up fine, hot water is very good to start with but temp drops over 10 min period. It is fine for a quick shower but you cannot fill bath right to top. I have tried it with the hot water only selected (c/h off) but it makes no difference. I thought maybe the motorised valve was playing up, but now I am thinking that maybe something else is wrong, any ideas?
Thank you

 

[Burnerman]

Initially you'll be using up the water stored within the heatslave tank. If it starts to cool off, there's a good chance that the diverter valve is letting hot water into the radiator system, rather than sending it to you!
Assuming that the burner stays on, of course.
John