Hot water but no heating.

[philpolish]

Hi all need some help please. As mentioned got hot water but no heating. The boiler fires up when you switch hot water on but not for CH. I have had a quick look at the pump and it looks pretty old. I understand that pumps normallu have a screw on the front but this one hasnt it has got a large black plastic nut on it.
Any help would be very much appreciated.
Thanks Phil.

 

[philpolish]

Thanks for your reply, yes it has a three way valve above it I have taken the cover of and the valve seems to be moving ok but it does leak when in a certain position. It is gravity fed.