Not really a DIY question however this is the most appropriate group i can
find.

I just had a new combi boiler fitted to replace an old system with boiler
and immersion. Problem is the water temperature varies quite a lot -
sometimes its blazing hot and give it a minute and it goes cold before
getting too hot again.

The guy who fitted it is really good (reputation) and am trusting him to
figure it out however i've got curious as to what it could be.

What could be the cause?

fwiw my house is a 3 bed semi 30 years old and no water supply problems that
i've noticed. New boiler is a Worcester.

Thanks

Paul