Right I have a 1.8 mk3 165k y reg and there's a fault with the heating. Basically I could leave it idling outside all day and it won't heat up at all no matter what setting I have it on, I drive it for a bit 5/10 mins and whilst driving it's fine but I say stop at lights and I notice the lights dim slightly when the revs drop back down to idle (on dash and headlights) and all the heat coming out goes again and it's blowing cold (lukewarm at very best).

Any ideas at all

the lights flickering / dimming would indicate a potential battery fault. If the car isnt heating up properly, that is probably a thermostat issue. Is the fan on all the time?

the lights flickering / dimming would indicate a potential battery fault. If the car isnt heating up properly, that is probably a thermostat issue. Is the fan on all the time?

Fans aren't stuck on no, seem to be working OK at different speeds have had alternator mentioned as potential cause but shouldn't be battery as it's a less than 6 months old rac one...

check the voltage of the battery whilst off and while the cars running that should tell you at least the battery health, alternator could be an issue but the distribution of heat is different, which is normally the thermostat!