Hot water running cold

[JAY123]

Hi all

Hope you can help

We have a Gloworm Ultracom 38CXI combi boiler.

During the Summer the taps and mixer shower run hot water consistently hot for as long as need be, but now during the Winter when the radiators are on the taps and mixer shower will run hot for only a few minutes and then suddenly run cold. This has happened now the past couple of years.

Any idea why this is happening would be much appreciated.

Thanks

Jay.