Have just tried Lawries's test. 500ml came gushing out the crankcase before I blocked off the drain. In short, it might be what we used to call "wet sumping" on old British dry sump bikes. The difference is, when those old bikes' crankcases filled with oil, you struggled to start them, and when you did, oil came pissing out everywhere, including great clouds of blue smoke from the exhaust. The XT doesn't show any symptoms like this - just the leak from the breather.
The old Triumphs, BSAs and Nortons had a valve to prevent oil draining from the tank back to the crankcase, which used to pack up. The XT manual makes no mention of such a valve.
The mechanic who services my bike spoke to old Yamaha hands in SA and they told him to be careful not to overfill the oil. This is the same advice I have seen a few times on this site. I am advised by these guys to only check the oil after running the bike for 10 minutes. So, this morning, I check the dipstick and it is absolutely dry - not a drop of oil on it. I ride the bike for 10 minutes, expecting it to seize at any time, check the level again and it is way over the max mark on the dipstick! I drain the 500ml out of the crankcase as mentioned above and the level is still over "max".
The XT manual says nothing about checking the oil only after running the bike and, in fact, for years (with the same mechanic) I've checked the oil "cold" and the bike has run very nicely without leaking.
So, when the bike is cold, the oil reservoir is empty. When the engine is warm, both the crankcase and reservoir have more oil than they should. And yet, the bike goes very nicely! Is this what they call an Act of God?
Lawries, if your diagnosis is correct, what do I do to fix it?
Geoffshing, beware the Tuareg wife, she is probably also a thumper with a few leaks!