Dear Customer,
With effect of the (insert date), our charges will increase from £X per month to £X per month.
We do not like to increase our charges, the last price increase we did was two-years-ago, but for us to maintain a viable business this is necessary.
If you would like to discuss this, or any other aspect of our service, please do not hesitate to call us on the above numbers.
Regards,
Your Window Cleaner
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Or words to that effect. I wouldn't mention anything about fuel price increases, inflation, the stock market, the retail price index; nothing.