My boiler pump fails to start/work when not used for several months?

I have a Potterton Profile boiler, with a Grundfos Super Selectric pump. If the Central Heating has not been used for some time (through the summer months for example), the pump fails to start/work when the Central Heating is turned on.
I normally get around the problem by firing up the central heating/pump (for a couple of minutes) every two or three weeks during the summer, which has stopped the problem from re-occuring. However, this year I forgot to do this and the pump is not performing once again.
I get the impression the pump "locks up" through a lack of use, so would be grateful if anyone can advise how to get the pump working once again?