Have to fill bath with shower from mixer

I have a gas fired Baxi combi which has been fine, and was recently serviced.

The bath taps are mixer taps with a shower option running off of them.

The bath tap runs hot only for a short period of time before running cold. To run a bath I have to use the shower option and fill it with hot water coming from the shower head.

Any ideas on how to correct this?

Try reducing the flow rate on the hot tap,ie turn it slightly on to allow boiler to keep the hot water flowing. Your turning your tap on fully and the boiler cant keep up.

I've tried reducing the flow rate from the tap but turning it to anything acceptable brings back the cold water.

I used to have no problem with putting tap full on, its only recently that this has happened.