Edit: The rules were changed in 2013 - the new rules are in this thread:
Tax relief on donations to charity
From:
Revenue Guide
WHAT DONATIONS QUALIFY FOR RELIEF?
The minimum donation in any single year that must be made to any one eligible charity must be of a
value of £250.
Cash donations made in instalments (e.g. Standing Order) also qualify.
There is no maximum qualifying donation amount save that where there is an association between the donor and the eligible charity/approved body at the time the donation is made, e.g. where the donor is an employee or member of the eligible charity/approved body, the relief will be restricted to 10% of the total income of the individual for the relevant year of assessment.
A donation must also satisfy the following conditions:
It must be in the form of money or designated securities or a combination of money and designated securities,
It must not be repayable,
It must not confer any benefit on the donor or any person connected with the donor,
It must not be conditional on, or associated with, any arrangement involving the acquisition of property by the eligible charity or the approved body.
How to claim
For a PAYE-only taxpayer, the relief will be given on a “grossed up” basis to the eligible charity rather than by way of a separate claim to tax relief by the donor. In other words, the donation will be treated as having been received by the eligible charity or approved body ‘net’ of income tax. In the case of a donation made by an individual who pays tax on a Self-Assessment basis, the individual will claim the relief and there is no grossing up arrangement.
Similarly, in the case of a donation made by an individual who pays portion of his/her tax under the Self-assessment system
but who also pays tax under the PAYE tax system, the individual will claim the relief and there is no grossing up arrangement unless he is not deemed a "chargeable person" for self-assessment purposes.
The individual PAYE-only donor must complete an “Appropriate Certificate” - see Appendix 2 'CHY2 Cert' -
and forward it to the relevant eligible charity to allow it to claim the
grossed up amount of tax associated with the donation.