Cheers all. I think I will email mr Nicol first try and see what he thinks, and if he has seen this issue before.
No, it is not an alloy collar, the top part of the tube (above the weld) is only ti, the weld seems to be at the junction of that part which is thicker and the bottom part of the tube, which goes down to the BB, and is thinner.
There is no light poking with a torch, as the crack is really thin. From the inside, the difference between wall thicknesses is obvious - there is a straight-edged gap which you can feel with the finger, just as if it was two different tubes end to end.
24p says you don't weld ti tubes end to end, so there might be an overlap - but I am not sure. The crack might even only be on the weld itself, but not through the whole tube, if overlap there is.
It it were a steel tube, you could say for sure that this is an external butt as you often have at the top of the seat tube - but of course in the case of steel the external butt is created in the tube mill, not by the frame builder welding it on. I don't know about titanium, but I'd be amazed if they welded two pieces of tube end on end as that would make a very weak joint, so it makes much more sense to think of it as a plain tube with a sleeve welded around the top of it.
Although the crack is in the weld, I'm afraid it does look as though it is headed right through the unbutted tube. Welding does weaken the tube underneath.
In some ways, Scot Nicol is the worst person to ask. Personally I would ask Vernon Barker for advice first, as they have no axe to grind, nor reputation to protect. And note on their website they specifically mention expertise in repairing cracked ti seat tubes, so maybe this is a not uncommon occurence and they could tell you all about it.
One thing's for sure, it is definitely worth repairing that fantastic frame.