The Ballad of John and Sandbanks

This week in 1969, John and Paul recorded what was to be The Beatles' final number one single, The Ballad of John and Yoko.
The song chronicles the events of 16 March to 1 April and covers John's marriage to Yoko, an episode that began in the car en route to Aunt Mimi's home at Sandbanks that John had bought for her four years earlier.
He called it "the most beautiful place I know" and had visited many times, initially with Cynthia and baby Julian, but also on his own and with Yoko.
Stories continue to circulate locally that John made a couple of clandestine visits to see Mimi after his departure for New York with Yoko in 1971. If he did return to the UK during that time he managed to keep it very quite. It seems impossible to imagine that, even when he was keeping a low profile following Sean's birth in 1975, he could have flown into the country and not been reported or photographed anywhere.
It could be possible that be arrived by boat, perhaps at Poole, or even Southampton - in a reverse of the events related in The Ballad of John and Yoko - but, again, could John Lennon, famous ex-Beatle, have enjoyed a quick visit by the shores of Poole Harbour without being noticed?
It's an on-going inquiry...
What is known about John and Mimi at Sandbanks - and some photos - is contained in Yeah Yeah Yeah: The Beatles & Bournemouth, which can be ordered here.
As for the recording session itself, that is extensively catalogued by The Beatles Rarity by the ever-thorough Happy Nat.