JAG IRS brake shudder

[ollykiwi3]

I have a JAG rear end with inboard brakes.
i have a really bad shudder/chatter when under light brake loads like slowing down to stop at an itersection or just generlly slowng right down, usually around 20-30mph and lower. it only appears to happen when brakes have warmed up a bit, and only during light braking.
If I push hard on the brakes its ok, i can push through it.
its not a warped disc, but it feels more like the pads gripping and letting go, or too much slop between the pads and the caliper body.
its like a high frequency "chatter" more than a shudder or out of balance vibration.
has anyone else experienced something like this?
Standard JAG XJ6 2 piston calipers. it also onlt seems to be coming from the Left side, but very hard to determine for sure.

any thoughts would be appreciated, other than the obvious of not slowing down
cheers