Richy,
Don't want to spoli this but be very careful offering sealing at this time of year. Sealing is so weather dependent, no damp spots etc, as any sealer will mess-up and we are now in mid-October, damp mornings not much warmth in the day etc.
Re cleaning price for that size area £1.50 minimum for that size, adjust if you have too, but try to get this sort of rate.
Re sealant of grass, DON'T.