http://www.ukba.homeoffice.gov.uk/policyandlaw/guidance/ecg/ecb/ecb17/

ECB17.1 Transferring a visa (or leave)

Where an applicant's passport expires during the validity of a multi-entry visa, the visa (or leave) may be transferred to the new passport at their request. Applicants may be advised that any visa (or leave) endorsed in the expired passport remains valid so applicants can, if they wish, keep their expired passport with the new passport and therefore do not need to request any transfer.

For transfers, the visa vignette in the expired passport must be presented unless the applicant can demonstrate that the expired passport has been retained by the issuing authority.  If the leave is being transferred, the old visa should be cancelled without prejudice by placing a diagonal line through it (to prevent any misuse). PROVISO case notes must be updated to indicate that the remaining validity of the leave has been transferred to passport number xxxxx.