Compact Air Con smell??

[Gavstar]

My Mum has a 52 plate 318i Compact which she has had for a couple of months. This week there was suddenly a smell emmitting from the air vents which she could only describe as like vinegar. The BMW dealer advised her that this is probably bacteria and the procedure for clearing it is as follows: At least once a week turn the a/c on to full chill for 10 minutes, followed by full heat for another 10 mins. This apparently gets rid of bacteria.
Does this sound right?? At least once a week sounds ridulous to us. They also offered her something to put in the air vents to clear it, which costs £100!!
If anyone has any advice it would be much appreciated.

>>> Edited by Gavstar on Thursday 20th May 21:03

[mjw]

It is bacteria & will grow quicker (& smell worse) the less you use the air con. You can buy a aerosol spray to spray directly into the vents. Cheap option but effective. Available from air con service workshops or a good Motor Factor