Mike if you want to match your hourly rate of £70 you need commercial, you won't get it on domestic, or at least I dont. Does your son not work for you now?
Any domestic I get I tag on to the end of a day for a lad and its a bonus, but I certainly wouldn't be doing it. Got one on Tuesday, front of terrace (not quoted yet), window cleaner would do it for £10 probably so what do you charge? Like you said to have a guy go, set up, use genny and wages I just know I'll be too expensive.
Commercial on the other hand, regular contracts and better money, last three commercial have averaged £1250 although it involved access hire. We have a few apartment blocks on regular contracts where one man can do it in 3/4 day for £220-£300.