Yep, what AlasdairMc says…it should be on your bank statement (on mine, with Barclays). If you're needing it for a bank transfer, you might also need the SWIFTBIC (not really sure what this is but it was asked for when transferring money from Ireland to England once).
Just checked mine, it goes GB46 BARC xxxxxx yyyyyyyy where the xs are my sort code and the ys are my account number. BARC is obviously for "Barclays" and the GB46 is obviously some kind of country code. I reckon you could google the IBAN code for your bank.
EDIT: Barclays actually have an IBAN generator on their website and they say that as long as you also have your swiftbic and iban, there's no charge for transfers less than €50000. Maybe your bank has the same thing on its website?
