The message
from Nigel Molesworth contains these words:

I still don't understand why it goes on and off every 2/3 minutes.
Someone earlier suggested the boiler could be 'recycling'. Do you know
what this means

It's "cycling", and it means going on and off every 2/3 minutes!

If the water is coming back too hot (without loosing heat into the
rooms) it can cause this.

ISTM that in this case the more likely problem is the opposite - that
the circulation is too slow. I wouldn't totally discount the notion that
the flow rate is too high but that is easily checked by testing the flow
and return temperatures at the boiler. As Andy pointed out earlier the
design criterion is for a temperature drop in the region of 11C but the
temperature drop would have to be much less for the boilers internal
stat to induce frequent cycling.

Either way if the boiler is not firing continuously then it cannot be
giving its maximum output. Given that the previous boiler at the same
rating apparently performed adequately then the radiators should not be
undersized which leaves flow rate to be investigated, sludging (and the
characteristic cool patch) having apparently being ruled out.

So what are the flow and return temperatures at the boiler?

Gas fitter not withstanding boiler malfunction cannot be ruled out
altogether but certain knowledge of the flow and return temperatures
should be the key to solving the problem.

--
Roger Chapman