worcester heatslave 12/14 oil combi boiler firing intermitently?

My Heatslave 12/14 oil combi boiler is being very tempremental and has been for some months. It has locked out nearly everyday for too long.(since last service actually)
I've got to know the basic workings(easy replaceable parts-photocell/jet nozzle) fairly well, but I cannot work out why it still locks out.
As the cold weather has drawn in, the boiler is firing up more often but only works some of the time.
When it locks out and it is reset, the boiler doesn't do anything. After a bit of a wait and another try, it will normally fire up as normal and work well until the temp is reached and the boiler turns off automatically.
The same problem is then met when it has to start up again.
Any ideas as to the reason it only starts/fires up when it feels like it?