It may be a symptom of a Combi (combination) Boiler. A regular boiler has a hot water storage tank to supply hot water - a combi boiler heats the water on an as-needed basis and has no stored hot water available.
If there's a big difference in the temperature of the water going in and the temperature required on the outflow then flow rate will suffer. There will not be enough time to heat the water to the desired temperature so the boiler reduces the volume of - it takes longer to fill the bath when it'scold outside.
Good Luck...
March 2016
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