it does not happen when I run the taps although the flow from the bath
hot tap is quite slow.

If the bath tap is fine, then I'd eliminate the possibility of a faulty
shower mixer before suspecting the boiler.

The bath tap is likely to be slow, as it appears that the installer
correctly backed off the hot tap isolator to prevent you drawing more water
than your combi can supply. A 24kW combi won't provide a high flow rate and
will not fill a bath quickly.

Christian.