Bath hot tap flow

[Jonty44]

Hi, hoping someone can make sense of my problem and point me in the right direction.
The hot tap flow on my bath mixer increases if I turn on the bathroom sink hot water tap, close the sink tap and flow decreases again. The bath tap supply comes straight from the combi boiler and the sink taps follows on from the bath tap pipes withe that feed ending at the sink. No other taps in the house have the same issue.
TIA.

 

[John the plumber]

Has this always happened from when it was installed or has this just started happening?

 

[Jonty44]

Hi, it’s always been like it since the combi boiler was installed (at least that’s when I noticed it)