Id have asked him.. "Are you actually going to do it yourself every month, 12 months of the year? winter/summer etc.. "
£40 is a good price, and window cleaners have to take into all sorts of overheads to be able to make a living.
It would have gone down the same road, as the usual story... wife said, "no thanks the husbands gonna do them from now on" - 4 months later he's only done it once, and they are stinking, and you are asked back, with a price increase to boot! ..lol
I would have pointed him towards a expensive system like ionics