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Thanks for your question.
Apart from persisting with the informal methods that you are trying (eg. calling solicitors with whom he had a relationship, speaking to friends and family to see if they have any info or were asked to act as witnesses) you might consider the Wills and Whereabouts section of the Law Gazette.
All solicitors holding a practicing certificate receive a copy and each firm would read the W&W section each week and check against their records, then contact the person directly if found.
Here is a web page advertising such:-
If your informal methods come to nought I would give them a call to arrange an advert. It would usually be the firm acting in the probate making the advert but private individuals can do this as well I understand.
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