Hi Liz
You need to do a sight visit first don't quote before you have seen the job . that way you will be able to judge how long it will take . and how many staff you will need. also are you providing the all cleaning chemicals ect The price you are thinking is ok if its just you thats doing the work however if you have to employ staff you need to factor in tax , insurance and holidays £18 ph would be a good starting point.
regards
Steve