Only happens when pedalling, and sometimes doesnt happen at all.

However when it does I can feel the clunks through the pedals like its being transmitted down the chain, but the gears arent slipping as far as I can see. Its just clunking.

A clunk describes it quite well too.. it is just that a "clunk clunk clunk clunk"

Any ideas?

I had something similar (and actually my bike is doing it right now) its quite horrible cos you can feel it reverberating through the frame, right?

Turned out it was just a sticking bearing or two in one of my pedals. Oil em up or take em apart and clean em up, should do the trick Need to do so myself...

My thought as well crank or pedal bearing

Check the crank arms are tiziiight!

Quote from: soopahflyCheck the crank arms are tiziiight!

What him said

shall have a wee ganders. Think the rear bearings need repacking as the wheel wobbles a wee bit now. But shall have a wee ganders at the crank arms too

If youve pedals (like the DMR Vs) that have a flat on each side so that the surface that bears on the end of the crank arm isnt quite circular, you can get rhythmic creaking/pinking noises from that (I had this with my Cook Bros cranks). Solved with a pedal washer between pedal and crank.