Thanks for your response
Then there you have the answer, this mileage is not treated as business mileage and there is a high probability that there is no valid claim, and as for the other workers, they maybe making a fraudulent claim to HMRC, and will face tax due to be repaid back to HMRC along with interest and penalties when this claims are identified.
HMRC tend to accept claims at face value, and do not look closely at or even request employment contracts (where present) or check with the relevent employers, and this is where huge tax bills often arise from.
However, I would advise that you write to HMRC and state
1) You have no employment contract
2) That your job sees you working all over the country but also advise the business area you are usually responsible for.
3) That the employer does not pay business mileage for this travel
4) Whether due to the duties of your employment whether a valid claim is due, as you have out of pocket expenses over and above the normal employee costs.
I feel there is a claim, but it may be that it applies to travel outside of your "area" at th every least, but most employers are fairly ontop of which mileage constitutes a valid claim, but you have already advised that this employer is not as efficient as some others are.
I would quote the Employment Income manual http://www.hmrc.gov.uk/manuals/eimanual/EIM32368.htm
and ask for clarification.
Or make a claim but just claim for mileage outside of your area boundary.
I know this is not a full and complete answer, but the work you do, automatically was going to involve travel all over the country, and due to that reason, I can give an opinion that could be held up and argued with HMRC, but as we are just an advisory service and not acting on your behalf, with a client relationship, you need to satisfy yourself that any claim you make is valid.
If they agree there is a claim there, and what travel it covers, then you are also then entitled to subsitance, for any overnight stays, and food (as long as its of a reasonable cost)
Mark your letter and envelope to HMRC c/o of the Technical team, and use your usual tax office address to post to.
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