As ever one lives and learns... looking at the BBMF website PM631 has a Griffon 66 (well it says it was built with one but doesn't say what it has now) and PS915 has a modified Griffon 58.
Jacquard has definitely changed the engine, but I wonder if he's actually fitted a suitably modified 58 from a Shackleton?
No matter, they still sound great! But a sad reflection that I can't now remember what a Shackleton sounds like taxiing - the last few I saw arrived, displayed and flew off again.