OK people - I'm in a position to start thinking about my tyres.
I have had really good experiences with the Conti TKC80 Twinduro, but I don't think they are aggressive anough for the desert. For very hard tracks, or 50/50 with surfaced roads, these are what I would use. In 5000 miles I couldn't see any wear on the rear of my XT600. Amazing! I think a 140 might be the biggest rear (in tube format)??
Michelin Deserts are reassuring and extremely tough (almost puncture-proof), but they are also £160-180/pair - a LOT of money! I have also heard that they are very inefficient / slippery on tarmac. And pretty hard to fit!
Metzeler MCE Karoos are brilliant tyres, IF you are riding off-road. I had a pair on my Tenere and they were ruined in a few hundred miles on the autoroute. The carcass is almost as stiff as a Desert but the compound is softer; they come in a 150 rear. They are very good value for money, especially in Europe.
There is another Metzeler tyre that nobody seems to use: the "Six Days 3". It is toughened, but still lighter than a Karoo. The blocks are widely spaced, but not quite as large as a Karoo (FIM Endure rules mean they can only be 13mm high). Only problem is a 140 rear maximum size...
And finally, the Pirelli MT21. Seemingly standard fitment front tyre for Saharan veterans. Is it really that good? I have heard that they are extremely vague and treacherous on the tarmac, especially in the wet. I had a look at some, and the carcass didn't seem very robust to me...
For the ride to Tunisia I will be finishing off a pair of Avon Distanzias (brilliant tyres for the road). If there's anything left in them, I may even post them to Istanbul, to fit on the return leg.

This is what they're like:
Any ideas? Suggestions? Experience? I am riding a heavily-laden XR650L on this route:
http://www.pm-04.com/graphics/icons/blackmedmap.gif
Thanks people. Really appreciate it.
Lewis:
www.PM-04.com