So...
The car felt twitchy at speed. Having read a few threads I was about to book it in to have the tracking and geometry checked / fixed. But just before doing that it went to my local mechanic to investigate an unrelated (I thought) rattle.
He discovered that the lower rear wishbone bolt was a bit worn... you can see it.

The offending bolt was replaced.
... and the twitchy steering was fixed!
This bolt was allowing a really small amount of movement in the wishbone that it would seem was the cause of the twitch at speed.
This might be unique to my car but I thought I would share in case anybody was chasing a twitch 'ghost'.
The car felt twitchy at speed. Having read a few threads I was about to book it in to have the tracking and geometry checked / fixed. But just before doing that it went to my local mechanic to investigate an unrelated (I thought) rattle.
He discovered that the lower rear wishbone bolt was a bit worn... you can see it.

The offending bolt was replaced.
... and the twitchy steering was fixed!
This bolt was allowing a really small amount of movement in the wishbone that it would seem was the cause of the twitch at speed.
This might be unique to my car but I thought I would share in case anybody was chasing a twitch 'ghost'.