B5 heater problem

(W 2000 Audi Avant 2.4 SE ) Yesterday I found that the heater was hardly working - it had been perfect the day before. Coolant level was fine, temp guage showed normal, nothing untoward - but only a little heat. The fan seems to throw out plenty of air, but even on high only a modest amount of heat (but air con is ok on cold). I am told that there is a motorised flap in the dash, which sounds likely. Is this something that is fairly simple to access and check/replace myself? Or is it a dash out arm & a leg job? Any suggestions gratefully received.

Latest: had a diagnostic check today and no faults showed up (ie no flap motor fault showing) also seems to be throwing out a bit more heat so maybe slowly sorting itself out? Possible leaf debris-from recent storms- jamming up flaps? Partial clogging of heater matrix? Bit of a mystery so far but apparently not serious.