Trouble is there's a lot of choice and I don't know which is best (longest lasting, toughest and least water ingress)
Stonechip http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_188459_langId_-1_categoryId_165594
Black Waxoyl http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_172631_langId_-1_categoryId_165594
Underseal http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_179695_langId_-1_categoryId_165594