I'm pretty sure it's the DC-6. I've heard it recently at nght (from Ely) and it make my ears prick up to hear that big-piston sound at night... very rare these days though we hear it quite a bit during day in the air display season. We used to live in North Staffordshire some years ago and we often heard what I believe was the same aeroplane overhead at a higher altitude. Saw it a couple of times and it looked like a DC-6. Mrs Lowtimer used to say "There's the ghost plane"....