Vapour trail pair over London

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Afternoon all,

Just noted a pair of vapour trails in close-ish echelon starboard formation heading northeast over London from the Midhurst direction at 1200; they appeared to be fairly small, probably somewhere Learjet and 737 in size, and, no, they didn't appear on FlightRadar 24! They were probably less than a mile apart and appeared to be at the same altitude.

My colleagues and I are curious as to what they might be, if anybody knows and is at liberty to tell...

Thanks

Treadders