PSA - sometimes a creak needs to be investigated

A noise like a chain clunking into position you say?

Hmm, mines started doing that noise.  Will check the crank condition carefully.

I had this last year, as others have said, a known problem.

I am not a strong rider and tend to spin up hills rather than grind. My suspicion is that I used my Giant Defy through winter in some fgrim conditions - but when doesn't a British rider not ride in those?

As to the point of keeping checking, "bottom bracket creaks" have turned out to be wheel bearings, freehub bearings, cranks, headsets, saddles, dry chains and so on across my riding group, you need to do careful elimination.

The crank was easy enough to spot once of decided to look properly, grasping the crank and applying a lot of sideways force.

This week on a ride the shout of "Mechanical" went up, and a rider said he thought his bottom bracket had gone. I saw a lot of flex in the frame, but no clunking or odd movement. The rest of the group were preparing to remount, but it didn't make sense, so I looked back along the drive train then grasped the rear wheel which wobbled alarmingly, "Bearings gone" I declared, but then wobbling the wheel, I thought too much movement - broken axle? Looked again and the chain-stay weld on his PlanetX titanium bike, about 9 years old had entirely failed. He walked home.

The lesson is, don't guess, prove.

Ah, it's back.

Which version of ultegra is this one ?

I've had my right-hand crank sheer off just like that on three separate ultegra cranksets now - twice on the same bike within about 14 months. All bikes are well maintained, not its not a neglect issue. Dura-Ace also suffers this problem. Only way to guarantee it never happening is to switch to 105 which has a solid crank-arm and isn't two separate pieces bonded together (as far as I know). 

Bloody hell, that could have been really nasty. Van-man gets a bad press on this site so thank you for adding a little bit of balance by reminding us most road users are decent most of the time. 

If you're riding a bike with fishing tackle for gears what do you expect?

I've got a fairly significant click that I can't find the source of. Time to inspect the crank properly I think

Blimey, that could have been very nasty. Seems to becoming a common problem with these cranks, well explained in this video https://youtu.be/FkEkQV-zK0s?t=181.

What are those black dot stickers near the pedal end? Did you get cracking there?

Hope your injuries are not too bad but a for that chainset, in the words of Edmond Blackadder, it rhymes with "clucking bell". That could have been very nasty.