Many thanks Chris, that all makes sense! It is certainly hard to know exactly what was occurring due to my memory now, and what my understanding would have been at the time. Brother and I have always talked about the new equipment made our plane follow the waves beneath, it is now clear that we should drop that part of our experience from the anecdote!
Thread drift: does anyone know if BOAC VC 10s flew from London to Dulles airport or only to New York? I have a vague memory that our VC 10 flights always went to/from NY, and we went by Electra between NY and Washington DC where we lived.