Mpn
This sounds a touch strange, especially the need for bank account numbers. As far as I understand it, you don't really need to mention in your will where your savings are, its not legally required for the will to be valid and / or for the beneficiaries to receive their inheritance.
Of course, it may well be good practice for your Dad to let you know (in a separate confidential letter) where all his bits are but I certainly don't think he needs that information in his will.
I would ask the guy what his credentials are. As the number of people without a will is quite high, recently I have experienced an increase in the number of people setting up their own will writing business - put simply, the people who approach you do not have any legal qualifications and are definitely not solictiors, they simply take instructions and send these off to a will writing company who then post the completed will for signing and witnessing. The chap you speak of could be doing this, nothing wrong at all with this way of writing a will but he may well simply be mistaken as to the level of information needed on a will. Hopefully, there is a simple explanation.
Good luck with everything at this stressful time.