If one doesn’t want to go to Halfords then Amazon has a best seller in these silica gel packs, 2 x 1kg for 13.95, each pack can adsorb 800g of water. (adsorb and not absorb, see why the difference in the Wiki entry on Silica Gel…)
http://www.amazon.co.uk/LARGE-DEHUMIDIFIER-REUSABLE-MOISTURE-ABSORBER/dp/B00AZZB5UY/
As used in a friend’s ancient half-timbered Mini traveller that does not have air-con to dry it out.