Well remember subbies are self employed its up to them what they think is a good deal and depending on how much water they use so if its 4 hours and 400 litres that would work out to a days work and maybe water supply being at home would mean its gona be a days work , a days work in London for a decent Subbie has to be £150
Maybe someone might do it for £100 with a prompt payment as it is only 4 hours
And maybe he could top up water and jump on his domestic or something
Summary
Being down south I'd say £100 ain't bad for 4 hours