Roy,
I think your T&Cs are clear and to the point from YOUR trading perspective. But two things bother me, and it's why I don't have T&Cs:
1). T&Cs cut both ways. You have 3 boxes to tick, 4, 8 and 12 weekly...but what happens if for whatever reason eg: sickness, weather, equipment failure, holidays etc you are unable to fulfill your contractural side of the commitment? Your T&Cs do not explain what will happen in that event.
2). Realistically, how enforcable are the T&Cs you're asking the customer to sign? Probably with a good contracts lawyer coupled with a court appearance and a judge sympathetic to poor window cleaners you'd probably win... and gain what exactly? £30, £30, £100....er!
If, for arguments sake Starbucks approached me and asked me to regularily clean all their SW London coffee houses it would entail me sacking 2/3 of my usual accounts and therefore my solicitor would be drawing up a contract to safeguard me should Starbucks reneiged. And I would have no hesitation in taking them to court! But a contract signed by 73 year old Mrs Smith from Church Road, Slough

Come on! You're not gonna sue her...so what's the point in T&Cs for residential?
If I asked any of my regulars or new customers here in SW London to sign a contract they'd laugh in my face!