Workshop just phoned with a quote of Au$1013 (fitted)... Harmonic Balancer and bolt $642, Belt $211, Crankshaft Main Seal (while pulley off) $160. Parts to come from interstate hopefully tomorrow or next day, fitting will take an hour or so.
Not a common BMW problem apparently, but being an extremely hot day (39C) using aircon and being stuck on freeway for an hour due to road accident, possibly contributed to the molten rubber. No further damage is apparent (or expected) at this stage.
Updated in the hope it might help someone else sometime. Thanks for the replies, they put my mind at ease specially being the weekend.
UPDATE....HB quoted above no longer available (due to warranty issues), has to be genuine BMW HB for an additional $300....ouch!!