Ethel is quite correct, as usual.
This is copied straight from the Cheque and Credit Clearing Company's website, who operate the cheque clearing system:
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. In cases of disputes a cheque remains legally valid to use to prove a debt for six years.
In answer to goddwitch's question, you will find in practice that the bank will probably require you to obtain a replacement cheque if you attempt to pay one in which is dated over six months ago. This timescale is at the discretion of the bank, though, and it appears from terambulan's experience that NatWest are working on a three-month validity period.