I have just had a sudden thought. I am a teacher and at times am sent on courses or errands for my school. I claim petrol money for doing this, but on my tax return this is deducted from my tax allowance. Since I have had to spend this money for work and am only being reimbursed, why should I pay tax on the money? That means, for example, I spend £10 on petrol, the school gives me £10 and I have to pay £2.50 of that in tax, so being £2.50 out of pocket for using my car for work. (I am obviously not objecting to paying myself for driving to and from work, which everyone does)