I did this in March (before the tax year end) and in mid-May I recieved a letter from the very nice tax man saying that I could claim £45 per year for the last 5 years, and that from now on I should claim this on my tax return.
I did only ask for £45 per year, but that is a reasonable ammount (and that's what the letter I copied from ARRSE used...)
So it does work, and for the cost of a stamp, it's "free" money.
MATZ