Warmflow 70/90 Upstairs Hot Water

[Padwick]

Hi,
I've had numerous problems with my 70/90 combi due to frost and am hopefully down to the last one but I can't figure it out.

Due to bursts, I've replaced some pipework, the flow switch, the CH/DHW double selector switch, and the PCB/Relay over the past few days.
At the moment I have the heating coming on fine and when I turn the tap it switches the DHW pump on and gives me constant roasting hot water to the tap in the kitchen downstairs. However upstairs at the bath, the water comes hot for the first 2-3 seconds and then settles at what could be described as lukewarm at best. I wouldn't like to be getting into a bath full of it anyway.
Looking at the system, I have the two valves going into the house; one which is downstairs radiators and one which is upstairs radiators, but the water just seems to go in from the pump and is not split up/down.
All the radiators are heating fully (cannot be touched) when the CH is on.

ANyone ever come across this before?

Thanks.

 

[Puddle]

What is the basin hot tap like,could be just to much flom rate through bath tap,try tuning down or if problem with basin,do you have a thermostatic shower fitted as could be cold flowing through a faulty non return valve

 

[Padwick]

What is the basin hot tap like,could be just to much flom rate through bath tap,try tuning down or if problem with basin,do you have a thermostatic shower fitted as could be cold flowing through a faulty non return valve

Thank you very much, you hit the nail on the head. Once I turned the tap down from full blast (the basin was always ok), I got piping hot water at the bath. Cheers.