For £16 plus £6 delivery, it would have been less but some absolute git made a bid with 12s left on the auction that lifted the price from £10.60 (!!), I have 25 litres or about 5 gallons of the stuff.
It got good reviews as a spray-on, wash-off product used at car washes across the country.
Spray on might pose a problem... So something like this looks the answer.
On eBay for about a tenner. Will check out the hardware stores at the weekend.