One hot water tap only gets warm, others in house get HOT?

Hi

I have a Glow Worm Compact 100p (permanent pilot) providing CH and DHW. All the hot water taps work fine apart from the bath tap, which when opened fully only ever gets warm water from the combi. Another tap in the bathroom, the sink tap gets HOT but only when run alone. When the bath & sink tap are run together, the water from each is only ever warm...

I'm a bit confused. Can anyone explain why this is happening and how to fix it?

Thanks,
uk-Pedro