Although I moved away in 1977 the Kar-Air swing tail used to depart most weekday mornings at around 0700 and by that time was about the last you could here of the heavy pistons at LHR . As I grew up close to the airport in late 50s and 60s it was a very very distinctive and emotive noise, ona quiet mrning, and 070 was a quieter time then than now , it took some time for the noise to fade away too. Being a nostalgic sort I was thrilled to get to Blackbushe earleir ths year to see and hear the Eagle 6 arrive and depart.
As to the last passenger DC6 at LHR there were Martinair and SAM ( an Alitalia offshoot ) still running in the 60s and I think one of the two Icelandic airlines used pax 6s pretty late.
These must have also been close to the last pax heavy pistons at Heathrow s well since they were reliable and relatively economic aircraft they probably outlasted all the different Constellation variants and the DC7s. Possibly a DC3 from Cambrian or someone might be a contender but it must have been a very long time ago now that anyone walked up the steps (not sure props and airbridges mix) of big radial engined airliner at Heathrow
PB