Bath taps are 3/4" bore and the flow of water out of them will be more than the basin or the kitchen sink taps (1/2"). As the flow of water through the bath taps is faster the water moves through the boiler quicker not staying in the boiler as long and so not heating up as much. This coupled with the drop in the incoming water temperature will result in the water being not so hot. Turn the flow back slightly and the water should get hotter if it does not then you have a fault with the boiler and should call a gas safe registered engineer to look at it.
Anygasman
December 2009
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