Main Eco 28 133 error?

Had 133 error code come up.
Nothing doing when trying to reset.
Shut whole thing off at mains, took fascia of boiler off to have a look to see if there was anything obviously awry with ignition electrode (had read that might be a problem). But no obvious problem (to my untrained eye).
Turned system back on and pilot light ignite d and heating ran for a minute or two, but then light went out and heating went off again.
Have repeated shutting off and on again - with same temporary result before boiler gives up ghost again.
Any ideas what problem might be (and yes, I'm a rank amateur!!)?