you should of said
sorry were booked with regual customers , but I do have one space left on Thursday morning at 750am . it will be 55 pounds payable on the day cash 
To be honest I don't mind doing one-off/irregular cleans if the price is right for the rental sector and/or for private sector too that's if I can fit them in. There is plenty of call for this type of work if you are prepared to take it on as most cleaners only want regulars on their books so people phoning around can find it very difficult to find anyone local that is prepared to take the work on.
I quote everything even one offs but I price high and do them on the way to or from work when it suites us not them , they also have to pay on the day with cash , I also tell them this , easy money if the price it high