After reading some posts on here about price increases - some saying write an essay - some saying don't even tell them - I opted for a happy medium: Tell the customer if they're in, if not, their "Windows cleaned" slip says As of 01/04/20 your new price will be £xxxx.
I've always done a polite letter, your price is going up because of fuel prices, bank charges blah blah blah. So far with the minimalist approach I've had nothing. No one has said anything. Nada. Zilch. Nowt.
It's the future.