Clacking noise as right crank goes 'over the top' once per revolution Every time the right pedal goes over the top I get a single clack noise with a little more 'travel', a point where the bottom bracket moves without any resistance, from that point in the rotation. I have only just noticed it recently and it seems to be more noticeable on the lower gears where the cogs are smaller.

Any thoughts on what to do or what might be causing the problem?
It's always difficult to diagnose noise descriptions, especially in the drivetrain.  But guessing by the:


  ... the bottom bracket moves without any resistance ... more noticeable on the lower gears ...


I would say the rear hub's freehub is damaged.  Pop it off the hub body and inspect where the pawls are (mind the pawl springs don't ping out!), look for broken pawls and/or mashed ratchet teeth (the teeth which the pawls engage with).  Post some pictures of it if you are unsure.