Hi Mal,
There are a couple of ways of looking at this.
1) If you are running a limited company and paying yourself a salary as an employee, then as an employee, you can claim a mileage allowance from your business. I think it's 48 pence per mile. So if it's a 10 mile round trip, you could claim ยฃ4.80 from your business. You could therefore pass this cost onto your client. This is meant to cover the cost of wear and tear on you car, petrol and the cost for insurance (NB make sure that on your insurance you have business use listed - it shouldn't cost any more).
2) The other way is to think of the amount of time you take to get there and back, load up the car, etc, if you charge say ยฃ40 for an hour's treatment, and the round trip is 30 minutes, you could charge ยฃ60 as you have actually devoted 1.5 hours of your time to the client for the home service. You may have a cheaper 'travel rate' of say 50%, thus you'd charge them ยฃ50... but time is money, and if you were in a clinic booking them back to back, that time would be filled with a paying client.
3) I actually charge the same rate for both - I have a 'session rate', as I figure when I use my clinic, I pay an overhead/rent for the use of it - it's ยฃ7 an hour and I have to get there, which equates roughly to the cost and time of travelling. Just be careful if you're doing this that you are specific about how far you will travel included in the price, so that you don't end up out of pocket. If it ends up being more than 10 miles away, for example, you might start charging more.
Hope this helps you come up with some ideas that work for you.