£20 per hour for 7 hours a week! that's a great one and i'd try to keep it if possible. no chance you can take on someone else and get them to do it?
edit: just had a thought, is that £20 per hour per cleaner, or £20 per hour which pays for the 2 cleaners, so actually £10 per cleaners hour?
if not the best excuse imo would be to say you're going to soon stop doing the work so you can concentrate on running the business so will slowly get stop your domestic cleans one by one, and that you'll stop theirs in 4 weeks etc. i did something similar with a few i didn't like and it worked fine even though we continued to clean for 2 of their friends for a further couple of months. if you're confident with what you say i'm sure they'll be happy that you're doing so well etc