Potterton Netaheat 1-16 MkIIF heats up water but not the heating?

It was working fine yesterday and today I put the hot water on and it fired fine, nice hot water no probs. But it won't fire up when I put the heating on. The thermostat is up enough for it to click on and the Honeywell electronic timer is set to on as well, but I'm getting nothing from the boiler. We only moved in a year ago so I have no history on whether it's been checked or serviced recently/regularly. I've bled the rads and no air came out though the small rad in the hall has never worked and it's stuck fast - can't turn it on or bleed it. No upstairs rads as this is a ground floor flat! Can anyone advise any checks I can make before I call a professional? I'm going to need laymans terms

Many thanks all

Chris