I've had my 4 wheel alignment and tracking done a few times but I keep getting wear on the outer edge of the rear tyres (one worse than the other).

Are there some bushes that need changing that can fix this, its not the make of tyre I've tried Continental, Pirelli and Nexen - any help would be appreciated. Geoff

Very interested in this thread as I have the opposite i.e. wear on the inside edge of both rear tyres. It was an advisory on the most recent MOT and the suggestion is that the rear tyres are toe out and the car needs 4 wheel alignment.

As a matter of interest, he tyres are Toyo Proxes. 2017 5.0 Autobiography Supercharged
2005 Merc SL350
2005 Disco HSE 2.7 oil burner

I had the same as you V8Noise -- one was though to the steel braid, from the outside looking in all seemed good with in excess of 5mm tread on both Current: 2011 FFRR Vogue SE 4.4 TDV8
Previous: -
2004 FFRR Vogue 4.4 V8 with LPG
1997 P38 4.0 SE with LPG
1997 P38 4.6 Vogue with LPG

I would suggest replacing upper and lower rear hub rose joints along with the respective bolts and washers. The upper joints have eccentric washers on them that adjust the camber angle - replace these, along with the bolts so the camber can be adjusted easily, then get the rear aligned again.