Well having brooded overnight* on the skew dovetail cock-up (see comments below) I've come to the conclusion it's quite funny as well as bloomin' irritating. The latter because I'm used to making mistakes of the wrong-side-or-the-line, whoops-how-did-that-chisel-slip-there variety, but it's really, really annoying me that I got the darn joint pretty nearly bang on the money for fit - just the wrong damn joint!
And that leads to the funny bit which'll probably stopped me picking over it all night. I've long bemoaned the fact that my theoretical grasp of woodworking has been so much better than the actual tool-on-wood skill, not that actually says a lot 'cos it's all relative... But this time I brilliantly blew the thinking bit and didn't even pick up on the evidence that pointed to the explanation (oh, it's obvious now ) but seem to have got the flip side of the cutting right. So is that it now? Will I be wielding the saw with precision for the next 20 years without a clue of how to work out where to cut with it? Actually I'd take that without hesitation.
Oh, and a lesson I almost certainly won't have learnt. Don't change what you're going to do half way through the job. It was going to have just splayed sides at the beginning, and the pins would have been right then. D'oh.
Incidentally, did I mention I got another wad punch on Sunday...? Rather odd - it's clearly stamped "12" but seems to be about 3/4". That make any sense to anyone?
* If you think I agonise over whether I'm going to do X or Y right before I start, you do not want to know me when I've got X or Y wrong.