You need to look at the facts, how keen are you to do the job now that you will be earning £1300,00 (162.50 @ 3 mths) less for the job per year then when you first priced it

. Myself i would not do it for the same monthly price that you quoted for a 3 monthly clean. If they asked for a monthly price that is what you gave them not a price for a quaterly clean.
The next thing they will do is say bye bye after the first clean and get another cleaner in 12 months time!
Been their, fell for it , never again. I put a price in with a little room for a discount, if they say no i move on.
O yeah and then they pay you 60 days later.