I too would recommend completely dismantling the heater and doing it properly. You won't believe the amount of crud that you have to flush out. It will also make your heater more effective and probably squeak less (ironically making one think that it's less effective cos you can't hear it as much!).

I now have a new leak into the passenger footwell, I suspect because I got a prang on the passenger front wing, denting the headlight ring and slightly buckling the front wing. I can't quite work out where the water ingress is.

Leaks turn Amazons from 'wow' to 'euugh'!

Tom