Hi Dan.
hmm i was talking to a window cleaner today who was in a similar situation well he built the round up 10 years ago and he is doing say £15-£17 houses for about £8-£10 these are semis bay fronted side windows small bay up and down at the rear too fair amount of glass indeed.
clearly under priced thing is they never ever dream of canceling and he also has many in a row also that said still under priced.
these houses must have been the first houses on the round, clean them say 3-4 times to establish your self as the new windy and get your investment out of it, then it would be your call worse case you take a 30% hit if they all cancel,
realistic you might be left with over 20%+ because alot of people would understand its quite cheap and the going rate is more these days.
i wouldnt rush into it thou take your time good luck with it!