hi pal, i had same problem so got british gas out, basically my boiler is italian and not designed to run baths, sounds daft but true.. these kind of boilers only have a small heat exchange for your water, e.g mine heats 6lt per min, the best out there heats 35-40ltr per min.
he basically told me to make sure to have heating on before i run water and open the tap fully then close it so its only running slowly.. this helps as you reduce the water flow, i.e getting more heat per ltr.
another thing he said is that when you run your water it comes from the pipes in the ground (usually frozen