There was a thread on PH a while back about warped discs - here:

https://www.pistonheads.com/gassing/topic.asp?h=0&f=23&t=1691393&i=20

And another (which I've just looked for and can't find) suggesting brake dust adhesion to be the cause for judder.

I had the discs replaced on my Roadster as the brake judder was horrendous - I seem to remember some improvement was had by re-greasing the front suspension.

+8 4.8