Hi Daz, my Z did the same light came on and off over long period and it turned out to be the ABS pump module, worth checking if you can't find the fault any where else, it transpired that it's a common fault on these Z3 modules made in the 80s the solder breaks down and causes the light to come on sometimes giving the code of any other part in that system i.e. wheel sensor. Worth knowing as I changed two sensors before getting the pump code. Pump from main dealer is four figures but you can get it repaired for less than a quarter of that and get a lifetime guarantee on the repair if it is the module. Hope this might help.
RHA, Roy.