although £35 might seem a bit steep if you think about the alternatives it works out ok,
1 - to save water use an aquadapter - they are more expensive and you have to change the spring every now and then
2 - use a ez tap, they are cheaper but you will have to be a careful worker to not break these (im guessing you would use more than three of them a year) also they are awkward to have in your hand all the time.
3 - don't use any control device or pinch the hose, this uses more water, even pinching the hose there are times when you have to let go moving around etc, and that extra water could probably do an extra £35 at the end of the 1 day
the aquatap is pretty robust, just a couple of ceramic plates, its light and trouble free (I have broken one however seem to be in the minority here as I don't think anyone else has)
in one van I have an aquatap, an aquadapter, and one pole where we pinch the hose of use with a backpack so you can switch it off, the aquatap is the best device on 90% of the windows, the aquadapter works better on high windows above two story and windows above garages etc, the hose withought a tap is only like that because I haven't got round to replacing the aquatap that I broke.
this is how I have them
18ft - aquatap
22-27 ft - aquadapter
40 + ft - pinch or nothing (not many of these really high windows on our round, supermaxs are really heavy with anything on the end at that height and I wouldn't like to put anything on an exteeme
anyways, buy the aquatap its an investment not an expense