Was in the BP garage (Dartford) earlier and bought my usual silver carwash. The lady gave me a card which she stamped and wrote "S" (for silver) on.
Buy 2 and get the 3rd free ! I assume she wrote the S so you can only buy like-for-like washes.
Current carwash prices are:
Silver £4.89 (wax and dry)
Gold £6.89 (wax, dry and under chassis wash)
Offer closes Dec 2009, at participating outlets only
Free carwash to be claimed by Deb 28th 2010