Central Heating problem
 

I have a problem with our oil fired heating. It is a closed (sealed) system and the problem is that the central heating is working but the water heating isn't! The boiler is a Trianco and seems fine. There has recently been a lot of air in the system but bleeding has not cured this. The pressure guage is showing pressure as 0 to 1, which I believe is correct but I have just noticed that the relief valve beside the boiler is letting drips of water out. This I thought would be due to high pressure?
Has anyone any idea why this would leak if the pressure is low? I recon that this is where the problem is but don't know how to or why this is. Any help or thoughts on the matter are much appreciated.