A postscript:
I spent Whitsun week in Beddgelert, in a 9-bedroom house just over the bridge in the village - not just me, I hasten to add. For those of you, like me, who have only cycled the Caernarfon/Beddgelert road in darkness, and with somewhat dulled powers of perception, it is really a very beautiful stretch, going past a lake or two, alongside a narrow-guage railway, and with towering peaks to either side, none of which I was aware of the week before (OK I may have been vaguely aware of the railway).
I cycled back to Swansea, using mainly BCM and BC routes and can also confirm that
a) cutting the corner through Rhyd to Maentwrog may remove a km or two, but adds quite a number of verticle ms. There were great views though, back over the Aberglaslyn estuary/valley
b) the Maentwrog/Trawsfynnydd climb may be a lot easier if you haven't already done 350+ km/have had some sleep in the preceding 22 hours, but it's still a bit boring
c) service at the Tre'r Ddol cafe is quicker when you're the only one in there and it's 9:30.