Problem with Worcester Bosch pressure

Hello - some advice on the below issue would be much appreciated.

So I've got an unvented CH/HW system which consists of a Worcester Bosch CDI Greenstar 30 and a Ariston cylinder. The boiler and tank are about six years old.

A couple of days ago we had a big fall in pressure and water released from the relief valve drainpipe. The water coming out of the drain is cold which seemed odd if this was venting due to increased pressure due to heating. Is cold water normal? The manual warns of warm or hot water coming out of this pipe. When topped up again the pressure goes to just under 2 bar when the heating is on and then drops off to under 1 when not. I don't think the drain drips when the heating is not on H/W not so sure..

My assumption is I (or the plumber) need to recharge (or replace if necessary) the expansion vessel and pressure relief valve to restore the balanced pressure in the system. That looks like a big job from looking at the diagrams in the manual. How long would that take a plumber to do do people think?

Any advice welcome.

thanks