Ah, that's ok then I was answering the right question! I got my spline key from Cromwell, although they don't do an M7 size either.
This might seem like a silly question, but are you actually using a 12 point spline rather than a 6 point? The fact you call it a spline key rather than a torx key suggests you are, but I just wanted to check. I tried using a six point key initially, and that was the same. One size was too small, one was too large.