My reply was not meant to cover all circumstances - for which I apologise. I do not disagree with any of the posts above and yes you can claim back up to 6 years.
The only issue I would have with some of the advice given above is that you have to be able to provide proof that you did a particular journey. The problem with trying to claim for a trip by "she who must be obeyed" taxis inc to the station on a duty journey is providing the evidence. The copy of a claim and related payment slip is really the minimum. If some of you have claimed and either not had it challenged or won a challenge that is fine. Unfortuneately, some of this is down to the individual who is making the challenge.
You may be interested to know that HMRC is now a combination of a number of departments (VAT, Income tax). Whereas the Income tax people used to be fairly gentle and easy to deal with, some of the other departments were more like an unleashed Rotweiller. In the new combined organisation it seems that the latter predominate. If you do have any dealings with the new organisation you will probably find them more difficult to deal with and convince, and more by the book (ie jobsworth). There is no doubt that the treasury have given them instructions to collect more taxes and you seem to be less likely to get a ruling to go your way unless you have a cast iron case backed up by a paper trail.
Which brings me to RolandPulfrew's question. There have been a number of cases (and at least one of which I have direct experience), where the taxman has not sent out tax forms to those expecting them, or has said there is no need to fill one in as your circumstances are straightforward. This usually means that if you fill one in you will be entitled to a refund (ie you are paying too much tax). This is another ploy to get more tax from people. My advice would be that if you are a higher rate tax payer you should fill in a tax form. Look at your tax code. Everybody is entitled to £5035 tax free income in this 2006/7 tax year, so your code should be at least 503. If it is lower than 503, you are not getting your allowance, which could mean that they assume you are a 40% tax payer with additional income to your pay (ie savings, bank etc). How would they know how much - and in advance when setting your tax code. Do you think the tax man is going to allow you to get away with paying any tax? I think not.
Shadwell