10mm front, 12mm rear. Thanks again to Damian at DPM.
Painted my callipers and added some chrome trim to the grille too (sounds a bit chav but looks great) but my camera run out of juice before I could get any pics. Will get some more up next week.
I would be able to go another 5mm at least.. I think you would be safe with a 10mm spacer all round.. I have modified my arches a fair bit all round though, just removing any spare materiel from the rear bumper, tucking the front arch liner etc etc..