Comet XV147 was built as a Comet 4C and made just one flight in 1965 from Chester to Woodford where it was converted to the second Nimrod prototype and made its second flight 31st July 1967, although it always retained the Avon 525 engines. It went to Pershore on 23/12/75 for Nimrod avionics research, then to the A@AEE.
So this airframe was very likely the one mentioned in post #4