hey sam,
If you are going to do the backs of these type of houses, you will have to arrange for people to be in so you can go through the house, dont even think of getting a trolly system for this, they are far to bulkey and a pig to get through houses and carpets ect especially with 25litres of water on, what you need as said before is a backpack, these are an excellent piece of kit, you dont want to wear it on your back, just carry it through the house, i just got one of the gardiners ones yesterday and they look good, the window cleaning warehosue ones are also great as well, you can strap them with a bungee to a sack barrow from ebay for all other types of work.
You will go through loads of water at first but as you go on you wont be using a lot at all, a backpack can clean one standard counsil house.
Get a push fit tap from gardiners to control your water
or an aquadapter, (better but expensive)
dont forget your connectors ez snap male and female will do you you can have the correct end put on for you if you ask over the phone when getting your backpack