Forgive me if you already know this. Your Grandfather is mentioned by name on p502 of Handley Page Aircraft since 1907 by C H Barnes. There is no detail of the accident but the text provides some context.
I would expect there to have been a unit enquiry into the in incident which should, by now, be a matter of public record. You may be able to find this in the national archive at Kew.
Good luck with your search.