Let me get this straight - he owes you for two cleans totalling £16.00, you use ten litres of his water to conscientiously finish a job as you have run out (or was that a mis-type?) and he "goes off on one" when you try to explain?
Yes - for all the pedants out there you have technically taken his water without permission.
So go and get a 25 litre barrel, fill it with water, put it on his front lawn at daybreak. Knock loudly on his door and say in a loud voice "I've brought your water back with interest. Please would you settle your account immediately?"
If he doesn't pay make a scene so that his neighbours hear everything.
But don't connect his tap to a hose and put it through his letterbox in the middle of the night:- that would be wrong and criminal.

Modified to add - Blimey Shiner,

we must have similar backgrounds!