I would go with renting a small van and using water barrels and a backpack.
This is one of the reasons why I bought a backpack so that if my van with system is off the road I can still work in some capacity.
That was my thinking too - as well as being able to get to the odd window where it's tricky to run a hose.
Anyway, I've bitten the bullet and hired a Transit 260 for the week. It's £158 all inclusive. I think that's an OK deal. They didn't have a smaller van available but the Connects are the same price to hire anyway. If I get my Trafic back a bit early I will just take their van back early too.
It's only a mile from home so I will take a stroll down there in the morning to collect it. £8k a year without maintenance and insurance actually sounds OKish. Obviously diesel is extra. If it were permitted to kit out a van mount in there, I might even think about that - especially if it were possible to get an enhanced discount for long term hire.
By the time you factored in tax relief, I wonder if it might just be worthwhile.