What Brian said 
I've made a number of both Vertex and Stratus which use the same mechanism. I use a Stratus dressed in lignum Vitae as my daily pen and it's beautifully smooth and trouble free. I also agree that the Beaufort rifill I put in this week is a definite improvement - thanks Phil.
There was another kit which I didn't like but had a mechanism which seemed as good but I can't remember the name so will need to look. My MIL nicked it and I think I had only one. It had a number of O rings near the business end! Someone posted one very recently.