They are Morgan specific but, as I said before Mulfab had their last set in stock a few months ago at £400 and something. Personally I would try skimming first at £100-£120
When the S1 was in production someone made drilled discs and 6 pot caliper conversions. I have searched fruitlessly to no avail for these. Perhaps someone on here will remember?
We made a 350mm 6 pot upgrade kit for the S1 at Mulfab, we only ever sold a couple of sets so didn't continue it.
The S1 front discs are Morgan specific and are one casting, with directional vents. This means that one disc is rotating "the wrong way" for its vents to work properly and this can cause overheating issues and warping. However, its more likely as mentioned earlier in this thread, that there is a pad shadow or pickup on the disc where the car has been standing. This causes an inconsistent friction surface as the pad shadow is more "grabby" than the disc itself. This causes brake judder and can feel like a warped disc.
I frequently bead blast the discs on the racecars to remove any trace of pad material build up, especially if I am trying out different pad materials.
A light skim should remove the problem
Simon @ SiFab.co.uk