Sorted. I post this to assist other sufferers - and those as anxious about changing a BB as I was - despite years of pulling engines to bits and getting them going again - so the mucky bits don't bother me. Had an intermittent knock from the stock FSA BB on my 2009 Defy 2.5 - done about 3500m. I could never find any play in the BB when rocking the cranks, but there was, but only intermittently, a definite knock from somewhere. Searched the forum (and others..), consensus seemed to be that this can be difficult to track down. Stripped and cleaned the headset, front and rear hub bearings, SPD bearings and cleats, seatpost, checked the seat rails, increased the lube on the chain, and gradually it went away. Hooray! Until yesterday. From the off on my 100k it knocked like a the proverbial outside loo door in the wind. Worse than ever - but it went away if I got out of the saddle. How come? It had to be the BB. But then it gradually faded and after about 30 miles it had gone - again. Got home and tried the crank rocking again - and this time there was a distinct movement - about 1mm at the crank ends. It was the BB after all. Read up and watched a number of YouTube vids on the subject and today bought myself a crank puller and BB key, and the Shimano UN55 -113 BB. I was worried about pulling the threads out of the cranks but, took care to get the puller in all the way by hand, then nipped it up, and wound in the pushrod part. It was tight, but never needed any more than a good pull on the tool - no hammers or extra levers required, and off they both came. Simples. The BB retainers came out easily - even after 3 winters of road salt, Cheshire cow muck, and frequent washing. All very undramatic. Took me about 30 mins, even including a good cleanup in the BB/chainring area. Just refitted a nice clean chain and cassette and given it a twirl. No slack cranks, knocking, or any other signs of distress. It's taken me about 3 months and maybe 500 miles to track this down - frustration isn't the word. I think that when you've tried everything else it has to be the BB - even if you can't find any slack. And for those out there as frustrated as I was, changing the BB is not difficult and well worth it. The old one sounded like a bag of spanners when I got it out - the new one is silky smooth. Very satisfying. Just the new middle chaining on the triple to change now, plus a new chain and cassette.......to go with last week's new Gatorskin Hardshells. Who said cycling was cheap?