Sorry Dave, just saw this .... right here goes.
Dear XXXX
I am writing to introduce myself to you as the new window cleaner taking over from ragit and run. I have cleaned your windows today at the rate previously agreed between you and Mr. Ragit.
However I understand that his rates have not increased in some time and due to ever spiralling costs - including inflation, bank charges and especially diesel fuel - I have no option but to increase the price to £XX as of and including my next visit.
I do hope you understand the necessity of this and assure you of a first class and reliable service. If you are not happy to continue then would you kindly telephone me on 01234 56789?
Kind regards,
DM@SI
(I don't know if I would put in the bit about phoning me tho' - I think I'd just leave the bit in red out and just put my number under my name.)