[embee]

The cheapest and simplest thing to try is brake cleaner on the disc and pad. I've found it will often solve the problem.

I get brake cleaner from the "Pound Shop", costs.......well, £1.

According to the various brake gurus, pad material does get transferred to the disc surface, and if this occurs in different amounts around the disc you'll get judder/vibration. This is often wrongly diagnosed as warped discs. Fitting new discs cures the problem therefore it must have been warped discs, right?

There are recommended procedures for conditioning discs and pads, basically start from clean/new and in succession progressively brake harder from faster speeds. They need to get hot. In cars the recommended procedure can result in smoke and discolouration (depending on how good the brakes are), but people will rarely be prepared to give their brakes this much grief. On bikes it's probably difficult to do this hard enough to cause a problem.

Note - this is only based on what I have read, so do your own research to verify/shoot down in flames.

I know there have been issues with production cars when disc castings have had the wrong trace element mix in the iron which creates slightly different solidification/crystallisation and thus different friction coefficients round the disc. Probably not relevant here.