I agree Daz. Just because you CAN get away with something, doesn't mean you should.
My letter is usually like this,
Dear ______________________
I'd like to take the opportunity to thank you for your continued custom and it is much appreciated.
However it has been several years since my prices have increased in your area which they need to do to keep up with inflation and allow for investment in replacement brushes, poles and equipment.
Therefore as of and including NEXT clean your price will be £______________________ and I hope you understand the need for this modest uplift.
Once again thank you for your custom,
Kind regards,