Thanks for all the advice!
Good news though - I've managed to find a 5DMkIII and it is due to be delivered tomorrow, together with spare battery and circular polarising filter
The only slight downside is that to get the 5DMkIII I had to buy it as a kit with the 24-105 f/4 IS lens. Is it a downside though? Actually, I'm thinking the 24-105 lens might be an excellent all round lens and perhaps all I need to take with me? I don't really want to lug too much equipment around, not only because of the bulk and weight of it but also the value for insurance purposes. Come to think of it, using just the one lens would also avoid the need (or temptation) to change lenses and expose the sensor to dirt, dust etc. So, do you think it's a good idea to travel with the 5DMkIII and just the one lens?
Regarding a ND/grad filter, I'm thinking of buying in to the Lee Filter System, unless there is something better/more practical?
One thing I think I definitely need is a tripod, for landscapes. I don't want to take my existing Manfrotto model as it's way too heavy and bulky to travel with, so can anyone suggest a good travel tripod that is light but sturdy and won't take up too much space?
Oh and the 5DMkIII camera strap is WAY too conspicuous - can anyone suggest a good alternative?
First thing first, try out you 5D3 kit set immediately in the situation you believe you'll encounter (low light situation, potraits, etc.). Since very few people have the body at this point, you have to figure out what it's really capable of.
I think the 24 on the FF body should be wide enough for most landscape situations, but you might be a bit lacking on the tele-end. Bringing your 70-200 or not would be you personal choice, since it will add significantly to your luggage weight and makes you very obvious when you pull it out. However, don't leave your 85 behind, because it well could come in handy for portrait/low light shots. An alternative is bring the 100 macro, which would allow you to go macro but sacrifice low-light capabilities.
If you really want to cover all the ranges, you can look at the 17-40 and 70-300 (L or non-L) to complete a budget travel package. Therefore:
Low encumberence set: 5D3, 24-105, 85 (or 100 macro) + flash
Ideal budget cover-all-range set: 5D3, 24-105, 17-40, 70-300 (or your 70-200) + flash
As for filters (I assume you mean the square filters), I haven't tried Lee so I couldn't comment on it, but I don't think there's much alternative left on the market (a pity that B+W doesn't have a similar system)
Tripods for travelling? Have you considered the 1-series of Gitzo? And don't forget the cable release.
Camera straps? There's a lot of choice in the market and it depends on whether you want the strap for carry comfort or the capability to wrap it around your hand+arm for a good grip. I use Think tank photo's strap because of it takes up less room and durable, and also works with my backpack (the 360).