Spy planes!!! Looks like I found an answer after two years when I came across a blog called 'secret bases'.
"Keen MI5 spy plane spotters should continue to lurk around RAF Northolt. A company formed by former elite RAF pilots, 2 Excel Aviation, actually based at Sywell Aerodrome, Northamptonshire, is now performing those spooky Northolt tasks, as part of its Scimitar special projects team.
They are using Piper PA31 Navajo aircraft "G-SCIR" (ModeS 40729C), "G-SCTR" (ModeS 40029D) and "G-SCMR" (ModeS 40729E) — all registered to 2 Excel in February / March 2017. They joined regular "sneaky-beaky" craft "G-UMMI" (ModeS 400C86) which is regularly spotted with special communications equipment attachments.
All three RAF Northolt Piper Navajo PA31 craft had been purchased by 2 Excel from Mike Jones Aircraft Sales Inc in Murfreesboro, Tennessee, America, who specialise in Colemill Panther conversions for performance and safety boosts. G-SCTR previously carried registration mark N331DB. G-SCIR had been supplied with tail number N522AW and G-SCMR had previously carried N27773.
Military aviation enthusiast Alan Warnes blogged on the subject in August 2017 on his Warnesy's World website, but strangely he only noted two out of the three Piper Navajos and missed G-SCIR. Curious, as the three registrations all took place at the same time – February / March 2017.
The London Evening Standard's Business Pages reported on my findings in an article headlined "Lift Off: Home Office privatises MI5 spy plane operations"."