I finally cracked and took it to a friend's shop, where by pure coincidence, they had a GT1 for a few weeks.
You're on the money with the cluster guess, although it seems to be a fault within the cluster. We decided to leave it alone, seeing as a new cluster is a dealer-only touch to the tune of $900.

Glad to hear you've narrowed down the issue.

The dealer will want you to pay full price however in the US you can call BMWNA and they will work out some deal to replace the cluster. For example you may only pay a percentage of the total cost, maybe only labor. May be worthwhile looking into?