David, as far as I am aware FlyBe Embraers do use ADS-B yet are ahead of their plots on FR24. I'm aware of multilateration and its limitations having been a base station for Planeplotter since the very early days of Kinetic receivers and the off track antics of non ADS-B equipped aircraft are well known so the theory regarding the number of contributing receivers is well made. I have used the tablet on FR24 in many parts of the UK, Hong Kong, Australia, New Zealand and the USA during the last 12 months but the deviations here are far greater than I've seen elsewhere.
The really interesting point is that the delay on approach for non ADS-B aircraft is totally consistent and seemingly not dependent on the aircraft speed or, presumably, the number of receivers contributing. The "offending" aircraft are always showing over the railway viaduct when in fact they are passing my location probably just under a mile further on.
The wandering Citations are odd. They always show up as unidentified Citation Excels and will either follow fairly standard routings for tens or even over a hundred miles, in seemingly level, but low altitudes and then disappear or they will just wander all over the place at varying altitudes before vanishing.
I put this query in the computer section in case anyone with an android tablet had come across these oddities. The type is an Acer A210, the service provider is BT and the download speed is a reasonably constant 20mbps.