Hi,
Sorry for jumping in straight after joining, but as unfortunately seems to be the way, I only find these useful forums when searching for solutions to a problem!
Anyway, I think I have a similar problem. My Wallstar keeps locking out and when restarted I can hear the air in the oil pump on the burner, but I don't know if it's a leak as such or just cavitating. It's fine until the weather gets very cold, then it usually stays running once it's started (so might lock out first attempt in the morning, but will then be OK all day), but now is locking out any time. So I'm assuming as the oil gets thicker it reaches a point where the pump can't pull through oil and either pulls in air from a leak or cavitates.
I'm wondering if the pump has just had it, but any ideas greatfully received! Just to add that I had the same behaviour last year when I found water in the tank filter which had frozen and blocked the line of course, but I got rid of that and now there's no sign of water/ice in the filter housing when it's cutting out.
Thanks,
Mark.