Thanks Remco - that was a good tip. After some digging I realised the issue was this line where somehow I'd managed to insert a point instead of a comma between the 2&3. I notice the current configuration dialog prevents the user from getting this wrong but perhaps it was left older from a change in an earlier version or perhaps (less likely) I edited the config file manually some time in the past.
<CPUCoresAssignedToVisualStudio>2.3,4,5,6,7</CPUCoresAssignedToVisualStudio>