i couldn't answer Hugh's question at this evening's meeting off the top of my head, but wikipedia to the rescue: https://en.wikipedia.org/wiki/ARM_Cortex-M#Instruction_sets

"All Cortex-M cores implement a common subset of instructions that consists of most Thumb-1, some Thumb-2, including a 32-bit result multiply."

I was much more involved with cortex-m SoCs in the earlier days but wasn't 100% sure of the instruction sets of the latest members of the family but, according to wikipedia, all cortex-m's use some variant of the thumb instruction set.

Best regards,

    Trevor