Back on a serious note, I would suggest that you don't tell them that there will be a price increase rather, if you feel you have to, explain that you are doing a wholesale 'review' of your business and pricing structure. Don't apologise or give reasons for the increase. It is rather lame to say that due to fuel prices rising they will have to pay £1-£2 more as the extra £20-£30 per day you would make on the increase doesn't equate to the actual cost of fuel.
If you say anything at all or give them a letter, you should express it in such a way that it appears to be the most normal thing in the world. If you grovel you sound like you don't think you should be increasing.
Before you ever tell the customers anything you have to be convinced that you should put your prices up, not because you can justify it with an increase in the cost of living, but rather because you are worth the final price. If you can't convince yourself of your worth without excuses you will never convince your customers.
Time for positive thinking!