OK, it was worth the detour. It's not 300 or 360, it's 410 Peter Wright I went up, he came down, sold.
Forward edge has been radiused for about 4 inches, I'm not worrying about the chips for now. No cracks or welds.
Addition: after some wire-wheel it's not marked ENGLAND and the '18' may be a '13', or they may have used a '3' and a chisel to make '18'.