I get asked this a lot, hence the reason for this brief post.
Capital allowances claims have a time limit, believe it or not. The absolute deadline is the second 31 January following the end of the tax year in question.
For example, if a tax year ends 5 April 2011, the deadline for a capital allowance claim is 31 January 2013. This, incidentally, is also the deadline for an amendment to a tax return.
The HMRC refers to the above in CA11130 under General: claims: Income Tax.