Just put it in today, with lots of much needed help from Mike. Its really quite a simple operation, and it all went pretty smoothly. The old one was an HT Racing kit, the new one is KTM SXS - performance parts from KTM. From what I've read and heard the HT kit is supposed to be a bit better, but you have to send you barrel to the US to get it done, whereas the KTM stuff is a straight replacement - you get a new barrel that bolts straight on.
I was going to take the top end off the old one, but after a bit of thought decided on a new one. It really is a much better bike as a 350, and although its the same weight as a 400, the lighter crank and close ratio gearbox mean it feels lighter, nimbler and more lively. I thought long and hard about what to replace it with, and came to the conclusion that I liked the old one so much I really just wanted another one the same.