Prop Freak, Antilles Air Boats had two that came across several years running as I recall - I think I saw one in the distance over the Solent while I was in Southampton in 1977... Certainly saw a Catalina with a magnetometer fly low over the house I was staying at that weekend, but that's another story!
Any chance of getting your pics on here?
Both Sandringham's wound up back in the UK in the early 80s - the other one is now in the Southampton Hall of Aviation.
Re the statue of Charles Blair at Heathrow, I recall a P-51C also as part of the memorial? What happened to it?