My car is a 1.8 as well. Mx5parts list the discs and pads for the mk2.5 1.8 *Sport* (this may be the crucial bit I forgot to mention) with different part numbers and prices than the other 1.8 models. The discs are almost £100 a pair, nearly double the price of a front set so there must be something different about them. Hmmm..."wonderbrakes"?
Time to do a bit more research I think... I'll be back hopefully with some answers later!
