Potterton Puma 80, Cold radiators?

Hi everyone.

I have had no end of problems with my Boiler, but i have fixed all of them myself. But this one seems abit weird.

The boiler works perfectly on both both HW and CH, but the radiators down stairs stay stone cold whilst upstairs they are nice and hot.

The pump seems to be running ok, the hot water is fine. I have bled the radiators and no air comes from them.

Does this sound like a divertor problem?

Thanks

Matt