I'm back from my trip so in case it might help anyone else with gear choices for similar trips, I thought I would record how things went for me. Please bear in mind this was NOT a photography focused trip, so what I say should be read in that context.
I took the 24-70 4L IS, 70-200 4L IS and Sigma 35 Art.
The vast bulk of shots were taken with the 24-70 4L IS. That is not surprising in general terms, but I was a little surprised at just how high the percentage was.
I found the IS on the 24-70 very useful. Shooting handheld inside cathedrals and other buildings, and trying to keep the ISO down despite the dim lighting, I was often shooting at 1/15 or 1/10 or even longer shutter times.
I am glad I took the Sigma 35 Art. I probably didn't use it quite as much as I expected, but I enjoyed using it and once I have finished going through my photos I think it will be responsible for more than a few of my favourite shots.
I was surprised at how little I used the 70-200 4L IS - and even some of the times I used it were as much because I felt like I should since I'd carried it as anything else. There were only a very few occasions when I really felt glad to have a longer lens with me, and most of the time I didn't feel motivated to put it on the camera. In fact, I'm now planning to sell my 70-200 4L IS - and probably pick up a 135L to use as a travel telephoto instead. (OK, that is something I have thought about for ages, but I think I will finally do it now.)
If I had had something wider than 24, I am sure I would have used it. (That said, I am not sure how much I would have used it though to be honest - but that might be just me.)
I used a LowePro FastPack 350 as my camera bag / backpack. That was a mistake - for walking around all day it just wasn't comfortable for me, even when I had limited weight in it (eg camera with lens, one extra lens, a few snacks, water and a few other small bit and pieces). Not sure if it just doesn't fit me well or if its carrying system just isn't up to scratch, but my back and shoulders got sore. I think next time I will get a camera insert and put it in a non-camera backpack.
I was a bit disappointed with my Peak Design Slide strap. I have hiked for days with my Black Rapid Sport and barely noticed the weight of the camera, but I certainly noticed the weight when using the Slide. That said, I didn't have the Slide connected to an arca swiss plate - I just had it connected to either side of the camera. That may have been a mistake on my part. I see a lot of people praising the Slide in another recent thread, so I intend to try it with one end connected to an arca swiss plate and see if that makes a difference to how the camera hangs and the way weight is distributed. (I didn't take my BR on the trip as I thought the Slide might have been easier to get on and off when walking around cities, etc, noting I only did 4 or 5 days of longer hiking outside cities.)
I didn't carry a flash - and I didn't miss it.
A tripod would certainly have been useful. I think I probably made the right call for me, for this particular trip, not carrying one, but there is no doubt I could have got better photos if I had had one.
Forgetting photography, it is really annoying that the local SIM cards where I was switched to international rates when I crossed a border! I was in France, Switzerland, Austria, Italy and England during a 3.5 week trip, and it didn't make things easy!! I have since heard that Orange may offer a SIM card which does cover Europe - something to look into.
I plan to post a few photos once I have reviewed them and hopefully found a few which are OK.