There is an allen head bold, but the head is within the body of the foot/clamp. I can see it, but there's no way to access it to adjust it without completely dismantling the clamp ( and I'm not even sure how I'd do that). The bolt goes up through into the silver bar where it threads through a long plate to hold the bar/clamp together. It should be secure enough that they won't come apart, but I'm more concerned about the potential for movement during travel which might make things "shake apart" - if that makes sense?. I'll have a look under the rubber strip, but when I was fitting them I don't recall seeing any access holes.
The bars came with the car from new a year ago and this is the first time out of the box - unfortunately bought from a dealer 100miles away, so a quick trip to replace them isn't on the cards.
I've put the roof box on and with this fixed in place it does seem to be more secure, but for piece of mind I might end up getting new bars for the next trip. Any recommendations on what to get?