Adam id say the symtoms definatly sujest the heater matrix.....its not a difficult job, just a little time consuming, the whole dash doesnt need taken out just the lower section of it.

It took me 2hours maximum, not knowing what I was doing, so Im sure with a little guidance you will have it changed in no time.

Personaly Id fix it ASAP as once the system is preasurised your temps will start to rise very quickly because of the air in the system.

I put off changing a heater matrix and cost myself a head gasket....once bitten twice shy and all that.