Is it allowed for a DIY job? I thought this was forbidden, or is that
only for a pressurised secondary circuit?

The primary circuit is fine for DIY. It is having large volume tanks at
pressure that call for special safety measures.

A boiler primary circuit just needs an expansion vessel, 3 bar pressure
relief valve, detachable filling loop and pressure gauge.

I'm not familiar with the design of pressurised primary circuits. Is
there some information you could point me at?

You can buy a kit that contains all the required parts which may even have
instructions. Fundamentally, you just plumb them into the system anywhere
that can't be totally isolated off from the boiler. The pressure relief
valve needs to be near an external wall. The pressure gauge needs to be next
to the filling loop. The filling loop needs a mains water supply. The
expansion vessel can go anywhere. Often these requirements can be fulfilled
in a single location, next to the boiler.

Look in the FAQ for futher info. www.diyfaq.org.uk.

Christian.