"They" have just flown past me, and it was definitely a singleton! The colour scheme matches G-CGNE. So I think you are right, two ADSB/FLARM/Mode-S transmitters onboard, one of which is mis-coded with the i/d of G-OHLI. I too have seen it before more than once, i.e. two aircraft appearing to be in very close formation on e.g. ADSB-X, but which turned out to be a singleton confirmed by Mk1 eyeball.