Starting April 2008 companies will be reqired to increase holiday entitlement to 28 days for full time employees - entitlement begins in October and only applies to organizations that do not already offer bank holidays in addition to the statutory minimum.
Please see:
Lloyds Employment Law Consultancy
There is also a downloadable document available there clarifying the legislation.
If you already offer 25 days holiday as well as bank holidays you are not affected.
Part time workers should receive pro-rata entitlements.