You might want to look at what the IR allow as max rates for mileage. Perhaps worth checking their website.
If you're paid less than the 'permitted' rates, you can make a claim on your tax return for the deficit. I do it every year.
As it stands at the moment, cars under 2.0l are allowed 40p per mile for the first 10k miles then 25p for the next 10k, and I think it then drops to something like 15p.
Hence if you get 25p per mile from the Service, you claim 15p x no of miles for the first 10k miles on your tax return
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