I shelved this yesterday as I had the first 35 clues all sorted and then hit a snag. I returned this morning and found that I hadn't made a mistake I identifying the numbers but that my speadsheet for 13 down was wrong (it had the wrong value for N)!! All finished and a clever puzzle. I didn't think that there was a lot of number crunching but filling in the border took a lot of faffing around.