Before 6 April 2008, if a company's seal was not used, then 2 signatures were needed for it to execute a document one of which must have been a director's and the other either another director or a secretary. From 6 April 2008, there is an alternative: a single director's signature is sufficient provided it is signed in the presence of a witness who attests the signature.
The crucial date is that of the last signatory. So long as one signature was made on or after 6 April 2008, the 2006 Act's requirements for signatures will apply.