Prima F60?

Have just moved into a new house and replaced the Rads for doubles as old ones wher 1970ish. Hot water getting around the system ok (bath and sinks) except to rads the last rad on system in the lounge is still cold have taken off all thero valves and left system to run but still no joy.and all air has been expeled from rads .Pump seems to run ok but may be 8 years old as far as I can tell. Pump is a Grundfos Ups 15/50. Does the system need a new pump and if so will it need a 15/60 higher head pump to cope. with double rad sysem.