Head for your nearest branch of Maplin Electronics. Their storefinder is here:
http://www.maplin.co.uk/StoreLocator.aspx?doy= 13m12
Take a look at the socket on the adapter before you go or (much better) take the adapter with you. (Unlike most electrical stores, Maplins employ staff who know what they're talking about. They'll be happy to help you find the right lead).
The most likely type of connection is the 2-pin 'figure of 8'. If so, you need this lead:
http://www.maplin.co.uk/module.aspx?TabID=1&cr iteria=mains%20lead&ModuleNo=1168&doy=13m12
Other possible alternatives are the 3-pin 'clover leaf' or 'IEC'. (See the links on the right of that page).
If you're sure that you know which lead you require, and you're not close to a branch of Maplins, you can order online.
Chris