Lifted from the co-op website:
What to do if a beneficiary can't be found
If a beneficiary can't be found, the first step is to ask the deceased’s friends and relatives if they know where the beneficiary might be. It may also be worth instructing a genealogist company to help locate the missing...
From a brief discussion I had last year (with a solicitor) there is no requirement in terms of means or method of communication - as the Executor you have an obligation to inform the beneficiary ‘when appropriate’ (usually as soon as practicable). Better to make the contact using text/whatsapp...