Don't get too worried about it. Sure it makes a mess but you won't loose too much oil. Mine did exactly the same thing at about 40,000km; had it fixed under warranty although the dealer did try to say the mods I'd done had contributed to it. Triumph Australia was OK with it though. The cam covers do have to come off as part of the major service and the gasket's pretty inexpensive, so worst case scenario isn't all that bad anyway.
In my case the cause was poor workmanship by the dealer when I had the head gasket done (under warranty). I reckon he'd just put the old one back in. And then, to make things worse, when he was told to replace it under warranty he merely stuck it back in with a heap of silicon. Of course it blew again, and that's when I worked out what he'd done - but that's another story.
My advice to you is firstly, insist on a new gasket (unlikely they wouldn't do that anyway but I never expected my dealer to do what he did). Secondly, inspect it carefully before you leave and make sure it's seating correctly with no bulges.
