You mention when running the bath, with no mention of HW delivery to other taps, first suggestion (if other taps deliver OK) is slow down the flow to the bath, you have to consider the time of the year and the relative temperature of the incoming water, generally speaking the bath tap is capable of pulling more water through the boiler than it can heat.
If however, all taps are likewise poor, then you would be looking to the DHW sensor, or blockage in the domestic heat exchanger.
Carefree, December 2010
