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.

I must say I have found our worcester combi a bit more awkard than having a
hot water cylinder. I find you cant really have the hot taps on just a
little as you need a certain flow before the boiler kicks in (so the water
stays cold), opening it a bit more and the boiler kicks in and the water
becomes mad hot, only when the tap is open about half way (a ceramic tap) do
I get a nice hot temperature that doesnt scold my hands! I cant say I have
had the constant hot-cold-hot... with a constant flow.