It's taken much longer than I thought it would, but I can run the testbench in the simulator and it successfully finds the (assumed) correct nonce 

I've only just learnt that 'midstate' is deprecated, which this code requires.  Perhaps I can work towards adding that feature (later, when I work out what this code is doing!).

The test hash I used was already in the code;


I didn't completely understand your example hashes. Am I correct in thinking you've provided data:nonce pairs, where one is correct?  Or are they all correct?  Obviously I'll need to calculate the midstate myself if this is the case.

Thanks for your help! I finally feel like I'm getting somewhere.