Thank you so much to everyone who has replied, I really appreciate it especially the wonderful newsreel and photo links. I have asked my father to check these to see if he can recognise my grandfather however we're pretty sure that Captain Pelly would have been inside the aircraft when Chamberlain was entering/leaving so is not visible. In fact in one newsreel you can see someone in the cockpit whilst Chamberlain is talking. We are also unsure whether he flew on all 3 flights but know for sure he was on the first one. Logs from this period would be very useful.
To answer your questions..
@Planemike - we have logbooks from his RAF days but not from BOAC sadly
@WHBM - Nigel learnt to fly in the RAF (Ulster 502 squadron which later became the Coastal Command) aged 17.
I was incorrect saying he worked for Imperial, we believe it was the companies I listed and also BEA.
I checked again with my dad and it seems Capt Pelly was actually the Directory of Operations at BOAC during the Berlin Airlift so we believe he was more involved in the organisation than flying.
He had to retire due to ill health in 1964 and died sadly in 1966.
@A30yoyo - thank you for the link to Allen Finch's book "Fishing to Flying" - I had already seen a reference in Chapter 16 but missed the one in Chapter 14 which was fantastic to read! I have made contact with Allen's son & wife and although he is no longer alive, his son has sent me copies of log book entries which show him & my grandfather flying together on numerous occasions.
@DeepestSouth - thank you for this reference, I have PM'd you to ask if this is the same photo as in Ian Thirsk's de Havilland Mosquito book (p. 373)
Once again to everyone who has responded, thank you! I will also send any further information I find out to this thread.