In reply to nickinscottishmountains:
> Still puzzled by the petrol, someone said to me that petrol costs are still expenses £45p/mile) even if they are refunded by the client.
Forget petrol expenses in this scenario; the 45p/mile is the amount you can charge for use of your private vehicle for business use, and is assumed to cover insurance, tax, fuel, servicing etc etc.
So, if you invoice a client £200 for a days work, and travel 50 miles providing your services, you can claim £22.50 vehicle costs as an expense. Alternatively, if you charge £200 plus expenses, you can charge £222.50, but only have taxable income of £200. If you charge the client, you cant then claim it as an expense (that would be, as you suggest, double counting).
If you charge more than 45p / mile, the difference between what you charge and 45p is taxable. Just make sure your car is insured for business purposes.