Ben,
I can't claim to have known your father-in-law well but our paths did cross on several occasions over the years and I always found Martin a pleasant, approachable and helpful man.
As far as I can recall I first met Martin when he gave me (subsequently along with many, many others), the opportunity to see a pallid swift in the hand that was taken into care, and was later successfully released. I also recall other events involving a wryneck with a 'wry' neck, and a great grey shrike, there were doubtless other such events.
Rest assured that birders have very long memories and various events, particularly those involving the pallid swift, will be recounted in conversation for many years to come and Martin will always be part of those memories even to those who did not personally know him.
On behalf of all those birders who benefited from Martin's willingness to share such events, please accept my thanks and commiserations for your sad and sudden loss.