In Asda Hereford today I noticed 1L Triplewax Car Shampoo was 100% extra free, bringing it to just £3.50 for 2 litres!
Cheapest I've seen online is £5.49 for 2L + delivery at Screwfix. Unfortunately I don't know if it's nationwide or store-specific.
At 15ml/wash, should be good for over 100 washes. Great for while the weather's hot, or indeed cold, providing you like having a clean car!