Shower hot then cold, but goes hot if basin tap is on

[jsm]

Hi
I have a 3way diverter shower, running off a combi boiler system. When having a shower hot then goes cold, but if I run the sink hot tap it then runs hot..
Any advice would be greatly appreciated.
Many thanks

 

[Gasmk1]

What boiler is it

 

[jsm]

Hi, it’s an Ideal Logic Combi 30.

 

[Last Plumber]

Is not the shower?

 

[jsm]

What boiler is it

Ideal Logic Combi 30 boiler.
[automerge]1738347166[/automerge]

Is not the shower?

Sorry, can you expand a bit more please, many thanks

 

[ShaunCorbs]

Ideal Logic Combi 30 boiler.
[automerge]1738347166[/automerge]

Sorry, can you expand a bit more please, many thanks

Think what last is trying to say if all the hot taps are working with no issue, then it could be the shower at fault