From there we tried browsing the internet, and again the connection dropped.
Very strange that it was only the two Apple MacBooks (Air and Pro) that were dropping. iPhones, iPads and windows PC were fine.
So we tried turning of the router (Time capsule) for 30+ seconds and turning it on again. When it rebooted the two Macs now reconnected and maintained consistent wifi connection - perfect! The helpful Apple support analyst explained that sometimes with too many wifi devices it can switch channels to compensate, and then get stuck on a channel. Hmm, not sure I fully understand that how that causes this. But anyway restarting the time capsule router has resolved the problem, thankfully.
I'm also doing an update of the other computer to 10.8.4 which includes an update of the time capsule firmware, which should ensure the problem has gone for good.