How to solve Vokera Linea 24 slow pressure rise?

I've had the above boiler since about 2001, and it's been working well. The only thing I've changed since is the pump.

A few weeks ago I notice a slow build up of pressure in the system, and I've had to routinely bleed off excess water to bring the pressure back down. My plumber suggested that there may be a fault it the filling-up loop, but after I disconnected this the problem IS still present. I would be very grateful for any thoughts on what is causing this, and what I can to do about it?