Seriously,.... Double your price,.... RIGHT NOW!!!
I charge €20 for the average 3 bed semi,.... and WFP I'd do 3 and hour in my sleep.
But that's if they are all in a row. You have to count travelling time,.. time between houses,.. unpacking your gear and packing it away again,.. it all adds up. 16 houses a day with ladders is possible,.. but very hard work if you'll only be seeing £80 out of it.
You might think that £160 a day is excessive (If you doubled your prices) but you have to consider the days you'll miss form illness, rain, when its too windy for ladders etc etc.
Don't undersell yourself,.. you're offering a valuable service, and ppl will be happy to pay.