glow plug light on.... 20sec... off = glowplugs ok (at least 3 glowplugs, ONE faulty doesn't show a fault)
glow plug light on... 20sec.... stays on (or goes out briefly then stays ON) the immobiliser has been activated (wrong key, door tried, or just a glitch)
what you describe is most probably a minor fault retriggering the glow plug relay.
check the relay connections... especially the thick red feed wire (it cracks at the relay)... stand in front of engine, looking at windscreen, the relay is down the right hand side of the radiator... the wires break and the fixing studs snap
it shouldn't be a problem