Chainring Failure (Bike Anti-porn)

I've got a 103BB which just about pokes out the end of the frame enough to attach the crankarms. The spiders have shoulders on both sides (which I think you can see in the picture - the one opposite the crank arm kinda shows it)

I'm guessing that the bolts just worked themselves loose over time - there's a stretch of cobbled stone road that I have to cycle through in Barbican every time I go to work. I've been running this setup since December and I cycle every day, so would have expected this to happen sooner if I'd done something majorly wrong, I'd have thought

Viewing on my phone so wasn't sure about shoulders.

As with many failure modes this one would have been progressive until it reached total failure but the failure started theoretically at least, as soon as the bolts weren't tightened sufficiently.
If there was a quiet part of your commute or you ride this bike on quiet lanes you would probably have heard creaking.

If it creaks check it.

You should check your chainring bolts regularly especially on a commuter.
Grease the threads and tighten up really well. If they're decent quality steel bolts you won't strip them.
Some people loctite them.

Normally, the only thing I hear is the rolling sound of my tyres... But yeh, I'm not a fan of creaking noises and usually check things out straight away. I was in a rush getting to work, and in fact leaving (plus it was full of traffic both ways), so I couldn't really hear anything or notice anything out of the ordinary. I shall definitely grease, if not loctite my bolts next time.

Considering getting a second hand Campy Chorus 53/39 going cheap, not no idea about chainline/how well it'll fit on the BB etc

C-Record era Chorus take a 111mm BB for a 42mm chainline; '95 onwards (says Chorus on the crank arm) take a 102mm BB.

Assuming it's the latter, you should be ok with your current BB if you put the chain ring inboard.