Intermittent boiler lockout - Main HE A heat only - help!

Hello, I'm looking for some help & advice troubleshooting a problem I have with my gas boiler. It's a Main HE heat only boiler so pretty simple, all it needs to do it fire up when asked and provide heat.

This is the second time I've had this problem (first time a few months ago - went away after a few resets), what's happening is the boiler fires up when the programmer kicks in, runs for for some time then I get a lockout and I have to press the reset button to get it fired up again.

Sometimes it'll run for quite a while, sometimes it go through the startup process and fire but it'll only run for a minute then cut out again.

Can anyone advise on a likely cause and where I should look? The pump and valves seem to be working oK and the condensate drain isn't blocked so I don't think it's there. There's no indication of a fault with the LEDs behind the panel which seems odd in itself?

Thanks in advance, any help gratefully received.