Since the Polyrace bushes from Rally Design were not a tight fit in the arms I had some problems with the rear arms moving under braking and acceleration. After a complaint to Rally Design they sent me a new set, but those had the same problem, after that they gave me a refund.
Then I tried some Floflex bushes, as I have never had any problems with those. But while they movement of the arms was less than with the Polyrace bushes it still didn't feel like it should.
So over to the best but expensive solution, the sferical bearings the cossies have from standard. As I didn't feel like buying 4 new ones (at 80 pounds each) I found a set of rear arms with OK bearings in them. At the end one bearing still had some play in it, so I bought one bush from GGR to replace it. Luckily the local garage was again nice enough to let me use their 15 tons press.
But now it's back on Sierra Cosworth arms instead of Granada Cosworth. I could tell immediately when looking at how the wheels sit in the arches, the rear track is a lot less wide now.