Just say, "sorry i cannot do it at that price...
You are better off sticking with your £4 window cleaner (even if he no longer does it - which is probably why they called you)
But feel free to contact me in the future if i can help you, but note my quotation is vaild for 7 days only.
Have a good day, sorry to have wasted your time"
Leave it friendly, say sorry even though they are to blame (in our eyes) it leave it open for them to contact you at a later date. And by saying the quote is valid for 7 days, it allows for increasing the price after that period, when you become more established and a better window cleaner.
stick to your own prices and pricing scheme, dont reduce just to get a job, unless there is something in it for you (long term!)