Not sure that it wouldn't involve travelling, unless the 90 are all in one block.
7hours @ £30 / hour sounds reasonable IMO, 210 for the day, plus vat, and prompt payment. There again, you can translate the number of apartments cleaned in a day to the £210 rate, in my experience, some apartments will have only a tiny carpeted area, then some will have 3 or 4 carpeted bedrooms,