Replacing a pump?

I have an Ariston Microgenus 27. It has required the pressure to be topped up about once a month for quite some time. Now it has started to cut out (lockout ?) with both reset LEDs lit. I have watched it and before this happens the pressure goes up to about 4bar then it cuts out. I can get it to work by turning off and on a few times.

The last time this happened the engineer checked the expansion vessel pressure and reckoned it was ok. He thought the pump may be to blame.

I've took the panels off and there is rust on the rear of the pump so that would explain the need to top up the pressure.

I have the service manual and replacing the pump LOOKS a straightforward task. Is it ? If so do I have to drain the system and how do I do that ?

Many thanks, Stephen.