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Okay; so on the new, clean install on the new, of the new, clean local account - it works! I moved it from D drive to C drive, and have been playing over there. I thought it might be the location, but checking again over here, it's still not firing up.

For obvious reasons I would prefer it on my main account, but I guess I can work around it.
 
That is an interesting finding, although it's hard to understand why a Windows user profile could possibly cause that problem ... !
 
It is possibly the least explicable fix - perhaps there is another programme on my main account which is preventing it from running (?) but regardless at least you might be able to steer others with this problem towards this ...well, solution of sorts.