OK, so I noticed the teeth were getting worn but I not want to change the setup before a big ride.
Yesterday I noticed the chain was jumping and I assumed with a few damp rides last week it was a stiff link.
I checked the chain over and it was all moving nicely so I thought I better try and find the culprit - a broken tooth.
I already ordered a replacement a few weeks ago and I managed to pop it on last night!
Thanks to
@Cycleops I managed to pick this up for £44 with a new BB included (did not use).