Right i've had this a while..

When releasing the throttle quickly at lower than 50mph the car shunts.

I've got a feeling it is the radius arms giving up..

Doesn't do it when accelerating!

Any thoughts?

could be lots of things but i doubt it's the radius arms , when the bushing goes soft they are generally steering issues , the feeling of drifting around corners having to correct steering in corners . is detectable by simply kicking the front wheels from back to front or front to back , they will move more than they should . having said that that doesn't mean its not them as they may be worn beyond the soft rubber stage, most likely a worn ball joint