One last huge thanks to everyone who responded with ideas. We had an amazing trip (despite the weather!!) and fitted in as many of the suggestions as we could. Wales truly is beautiful and some incredible routes to blat along. We'll definitely be on the Taffia fish'n'chip run next year if at all possible. I'm going to add one more route here in case anyone stumbles across this thread in the future: the B4407 off the A5 south of Betws-y-Coed down to Pont yr Afon-Gam is an awesome winding narrow road across open moorland. Quick, flowing, challenging, with lots of blind crests and corners, so it's a case of blat-what-you-can-see, back off, repeat. National speed limit applies and that's plenty for being hugely fun and not utterly reckless. There are loose sheep around but they seemed generally content to stay on the verge and ignore the cars - but still another hazard to be aware of. Enjoy!