Thanks for the advice, really appreciated.
Did it Saturday on my hybrid and it was definitely the right choice. Only had to get off the bike briefly on two occasions (only 50 yards or so each time) as bike didn't have low enough gears, once at the steeper bit at the top of 7 mile bank and again somewhere near Merthyr. When I get the bike serviced I will have a triple chain-set fitted to replace the double, to give me some granny gears and then hopefully should be able to do it all without pushing.
Took my mate and I 6hr 45min including two short food breaks, which I was pleased with.
Probably stop at Ponty, where we live, next time as the Ponty - Cardiff Bay bit seemed a drag, as we know that part of the route so well now.
Thanks again.