A cheque is legally valid for
6 years but, as indicated above, banks may not honour a cheque over 6 months old.
This is from the APACS (Association for Payment Clearing Services) website;
"It is common banking practice to reject cheques that are over six months old to protect the payer, on the basis that payment may already have been made by some other means or the cheque may have been lost or stolen . However, this is at the discretion of individual banks. It should not be assumed that cheques in excess of six months old would automatically be rejected � the only certain way to cancel a cheque is to request that a stop be placed on it (although cheques backed by a Cheque Guarantee Card cannot be stopped). It is recommended that, if possible, customers in possession of cheques that are over six months old obtain a replacement. A cheque remains legally valid for six years."
http://www.apacs.org.uk/resources_publications /faqs/cheques_10.html