Business mileage from other primary work to home, whilst doing our job

Hi, any advice you can provide will be grateful. We are a mobile mechanic business and require going to customers houses or work places to do our work. I have an employee who works for us between the hours of 5:30pm to 10pm and we are considered their secondary job. They do have a Primary job with another unrelated company which they work from 8am to 5:30pm. I need advice on how I would calculate the business mileage for this employee as he does our work while traveling home from his Primary Job and I am confused on the rule that you don't get paid for traveling to and from work or any normal commute. I can provide an example. Lets call his primary job as A, work job from us as B and C and his home address as D. To keep it simple lets call his normal commute from work A to Home D as 10 miles exactly. He leaves Work A and travels to Job B which is 2 mile away. then travels to Job C which is along the same route he would normal commute home. From job B to C is 12 miles, then from Job C to home D is 1 mile. Total he has done is 15 miles from Work A to B to C to Home D. This is where I am confused, in a normal day he would have to commute from Work A to Home D of 10 miles. So would I work out the total of 15 miles take away his normal commute of 10 miles. As he has only gain an additional 5 miles of fuel usage or wear and tear on his vehicle etc.. above what would be his normal usage OR Do I have to pay him the whole 15 miles, in effect paying for his fuel home. I hope this is not confusing for you. Regards

Hi
Please contact us on 0300 200 3200 to discuss this in more detail.
Thankyou.

For future reference and to help any other person in the same situation. I called HMRC and they advised that it is entirely up to our business if we want to pay for the mileage at all. Either 5 miles or the entire 15 miles or no miles, this is due to B and C being clients and the employee do not start at what is considered a work address. This leaves us with 4 scenarios. 1) Pay the full journey from work to home 15 miles. 2) Pay from B to C and not A as they are going to the job and not D as they are going home. So in effect Job to Job, this does not work where there is only 1 job involved (A to B to D) 3) Pay from A to B to C and not D as they are finished and going home. 4) Pay any additional mileage beyond the original commute mileage. So in the above example 5 miles.