them 2.50s soon add up
But at 5.00 pounds each, they'd add up to double what you've got now.
2.50 just about buys you a pint of lager; those prices need to go up to AT LEAST to five pounds each.
I'd draft a well-worded letter of explanation, telling the customer why you're doubling the price and that unless you hear otherwise, you'll be cleaning their house at the new price as normal. Give them a week-or-two-notice.
If you stay at 2.50 each, I'll give you three-months before you drop the lot, and then some other window cleaner will come along and pick them all up at a better price.
If writing letters isn't your forte; post your draft on this forum; and I'm sure we'll help improve upon it.
Also, do a search on this site for 'price increases'; it's been done to death, and I'm sure you'll formulate the best method for getting those houses up to a more realistic level of pricing.