As above, if you haven't checked the plenum since that last bout of heavy rain across the UK then do so,
ASAP.
Despite having had a recent full service by my regular mech - who knows to check the plenum drains - only a month or so ago. I've just found 1/2 an inch of water in the bottom of the plenum. I only checked as every time I've put the A/C on recently I've been getting a slightly off smell for the first 20 to 30 seconds of cold air before it clears, I had wondered weather the pollen filter had gotten damp: it looks like it must have got wet at the bottom. Evidently those last heavy rains had managed to wash some debris down to block the drain and start filling the plenum. I'm glad I checked it now.
The following links to the 'Spyhole mod' how to (I really should get around to doing this).
http://www.the75andztclub.co.uk/foru...d.php?t=100130
Maybe another member who know a good link can link to an explanation/how-to for how to check/drain the plenum?