For my own part, I work for a charity called Maritime Archaeology Trust. My principle day job currently involves studying 1,100 First World War wrecks along the south coast of England, but previous projects and work have covered D-Day (on both sides of the Channel), Landing Craft, protected wrecks, coastal change, Stonehenge, historic maps and the Second World War in the New Forest. Coastal Forces is a personal interest (for which I don't have any strong reasoning, I've just always liked the boats and been fascinated by their role in the war) and I'm currently building up a website: http://www.spitfiresofthesea.com
. I'm also quite well read on the Scheldt campaign and have cycled the area several times.
Added to this I have a terrible affliction that prevents me from walking past a second hand bookshop without going in. As a result I have a pretty large library, much to my wife's chagrin (but as she also works in maritime heritage and with historic fast boats in Portsmouth Historic Dockyard, she often finds it useful). I should probably have mentioned that, whilst I haven't read Dalziel-Job's book, I do have a copy and he mentions his time at Orkney there. That said I lent the book to a friend only last week.
As a result I'm quite good at making use of resources, published, in archives and online. There's somewhat limited information online about your father's friend, but I've found one possible source. The bottom paragraph of this page suggests he was a tea planter in Ceylon. A reference in bullet point iii a bit above that suggests he had children of his own in 1941, 1942 and 1946, so he's presumably the right age. http://www.marshalclarke.com/ClarkesOfG ... arkes6.htm
I have a friend who speaks Geramn, so I'll ask them if the translation of Doenitz's letter is correct. That's quite an artefact you have there!
As for the other survivor of 916, we've spent some time trying to find his name for Mark, but so far without luck. I'm sure the identity will be in a document at Kew, but I haven't yet found it. The identities of the lost crewmen can be found on the CWGC site. Type H.M.M.L. 916 into the unit field. http://www.cwgc.org/find-war-dead.aspx