Hot water from boiler filling expansion tank?

I have an old type CH/HW system with storage cylinder in airing cupboard and Bermuda 552 back boiler. I noticed that hot water from boiler going to pump (Grundfos UPS 15-60) also fills up expansion tank in the loft (there is a junction before the pump) causing the tank to overflow.

I tried playing with different settings on the pump but no joy. Anybody has any ideas what this might be? To me it looks like the flow of water from the boiler is too much for what the pump can manage thus some of the water goes into the expansion tank.

Thanks for help