Dan
As its only 25 sq.metres just set up with your rotary cleaner and away to go,wash down and clean up afterwards,you should probably look to charge about £2.50 per sq metre,and have a minimum charge of about £75.
Timewise from start to finish,probably one hour,so in order to give value for money do a pathway aswell or a wall or whatever,always look to exceed the customers expectations,its the best way to get referals,I am sure others will have different ideas,just take them all on board and you will find your own way
I also bought my equipment from merril fox,superb service-no problems,saved over £800 even with import tax,good luck Neil