Sounds like you've got it sussed! I'd agree with you that it is the heater matrix. I had the same on my last S2.

I would'nt say its a doddle to remove/refit, It just takes a little time and creeking off the neck, but by no means difficult.

From scratch to having completed the job for the 1st time 2hours max.

Its just the lower section of the dash that needs removing.