Turns out it was my bad!

For some reason, I didn't notice that the replacement suspension arm had an 18mm taper ball joint instead of a 21mm.

Despite it fitting and driving fine for a month. Sods law I hadn't tried to fit a 21mm into an 18mm taper and spotted my mistake first time round.

When the alignment was done, it must have shifted resulting in the ball joint becoming slightly loose.

I just put the right arm on - ECP site shows both arms for the same reg of the car but you have to dig into the details to find that they have different tapers on the ball joints. (which is also annoying)

There was a good online discount, so the new, new arm was only £88 for a TRW made arm.

You live and learn.

Protyre Poole were very good, doing the tracking three times (laser 4 while alignment, with new kit), for the price of one. They spotted the play in the ball joint on my second visit.