Small update.
Brought a pair of Audi 90 rotors from Upullit the other day. I went there just to find one to use as a dummy to get the correct calliper placement etc but managed to find a car with 2 new discs so well chuffed @ $56 for the two

Took the axles to work today and machined approximately 3mm off the diameter of the axle flange to enable the rotor hat to slide over. This needed to be a close fit as the centre hole on the rotor is 67mm apposed to the 63mm hub on the axle. This would ensure the rotor is positioned concentrically.
After doing this I trial fitted the rotors onto the axles with the original R31 brackets and found that (as mensioned earlier) the callipers are positioned perfectly for the offset of the Audi rotor. Pity the original R31 rotor has a diameter of 260mm - 15mm larger than the 245mm Audi item. If the R31 rotor is used I'm sure the original calliper brackets can be used as the rotors have a similar offset.
I did contemplate using the R31 rotor but I favoured the Audi rotor due to the already 4 x 108 PCD (and the fact I picked these up cheaply!).
I have drawn up a simple bracket from 10mm plate which should make a simple conversion.
I will upload images later as I forgot the camera [:I]