P87 Tax Relief on buisness mileage - Filed incorrectly help

Bracken

My brother-in-law has asked for my help as he has submitted a P87 and he can't understand why his tax code has gone down considerably!

I've had a look never done one before. I think what has happened is he calculated his mileage correctly but in the mileage allowance payments received he put the figure from his P11D which was for his fuel card not his car allowance.
HMRC have now added his Fuel card amount to his income as a benefit in kind and deducted the expenses which is his buisness miles less his fuel card amount. He has ended up with a large tax bill and a new tax code of 345L.

I have checked his payslips and he is paid an monthly amount in his salary as a car allowance. This is taxed with his salary but the company have not detailed this on his P11D in section E.

I think he should have deducted his car allowance payments from his payslips from his total mileage allowance to calculate his mileage allowance relief is this correct? And if that is right do you use the gross amount or the net as tax has been deducted?

One last question how best to get this corrected, have looked at HMRC website and I am thinking he needs to write a letter to detail what has happened, and I think it may be a good idea to include a SA100 with SA102, or would a corrected P87 be sufficient?

Thanks in advance for any help you guys can offer, trying to understand all this has certainly made me consider the benefits of ATT as my next study course.

T.C.

I would suggest the corrected P87 along with a covering letter.