Boilermate OV A class SP - showers turn suddenly cold

My Boilermate is approx. 10 years old. We recently (July) had two Mira showers fitted in en-suite and bathroom.

After the shower runs for a few minutes hot, it suddenly turns clay cold for about two or three seconds before turning back to hot.

Why should this happen? We've had the showers checked and both do exactly the same.

Out kitchen hot tap also runs very hot for a couple of minutes before going cold and then hot.

Any suggestions?

Also are there any Gledhill heating engineers in the Sunderland / Newcastle area?

Regards

Martin