From what I remember from previous posts Graham, it isn't a straightforward tube swap as there are all sorts of set - up issues to take into account. Unless someone can find a cheaper option then buying a good RF (real flat screen) used one 2nd hand works out cheaper than replacing the tube.
I'm certainly no expert so I could be proved wrong here but I believe the tubes were supplied by Phillips and that's about as much as I know.
Please let us know how you get on as there are plenty of these out there with this problem which is a bit of a shame as most people are intent on the newer flat panel LCD/Plasma sets, most of which don't come close to the picture of an Avant as yet.
Chris.
Forgot to add - @ £500 for a replacement tube from B&O was the last I heard - if they still supply them of course.
The use of metaphors should be avoided like the plague. They're like a red rag to a bull to me.