Hi Kev - just noticed this thread - been away on holibobs living the high life in my 24 year old caravan.
You've got a lot from the guys above to think on. But forgive me if I missed it - you said you were having tilt and turn windows fitted and you "only" have three of them?
Can you not do them from the insides now?
If I have missed something and you still want them cleaned externally I would say ...
Most other window cleaners that don't do shops/high streets as a niche will not be interested. Those that do will realise yours is a one off in amongst another window cleaner's work and realise there is little room for expansion. If you were able to get the other flat owners across to a new window cleaner at £40, then would it be worth it to him?
Probably not as he will have the issue of water dripping on shops below and either feel the obligation to clean theirs or feel the wrath when he says "not my problem". Or do them at a time when they aren't about to notice.
All in all I would do them from the inside if not then talk to the other flat owners and ask their view. If you are getting brand new UPVC frames then buttonhole the windy and ask "innocently" whether the new frames get done at the same time as the glass (if your old ones were wood or aluminium he may have avoided doing them for "milk" runs and drips) as you want everything kept up to scratch.
He will feel a little embarrassed if he has been avoiding top frames and assure you that they will be included as UPVC doesn't cause the same issues as the old non UPVC ones.
The price in my opinion is at the upper end of "OK" but 3 windows will always fall into the "minimum charge" territory which is often £10 - £12 anyway.