I contacted the IR to try and claim the difference between mileage rate paid by NATS for a couple of years and the amount you could claim from the IR. They promptly sent me Tax Returns for 5 years. The necessary paper collation and form filling took me a considerable time....so the IR promptly fined me for being late for the 4 previous years!
I ended up owing them a not inconsiderable sum of money and now have the pleasure of having to submit a return every year.
And I never did get the difference between the mileage rate and what you can claim!