Even easier let car idle so fan comes on then check as you know it is up to temp then, if still crap check pipes going into heater nice and HOT, if both hot try flushing, or disconnect pipes plug or join them and let car sit with bleach in overnight, flush very well next day, if still not fixed check BOWDEN then if all ok change matrix £30ish.
They are not the most awsome heaters in the world anyway, MK3 astra!!, my god they cook you well.
tabetha