i said that to her but she was adamant she wanted to ring me when they want doing next
Stand your ground. You offer a monthly or two monthly service at your standard prices. If they want to phone you as and when they need you, fine, but the minimum charge for that 'premium' service is £50
When they question that, tell them that wherever you are you will drop everything and come to do their windows.
For that they have to pay, and lots
I have one customer who is fine with that, the rest all declined my offer, which was the desired result.