Stephen , good thing you think about touring to Europe again for the Xen Summit.
Colocating with LinuxTag or LinuxKongress would really be a good idea, but make
sure to organize it the day before or after , not during the congress :)
On the other hand .. you wrote LinuxKongress, but you probably meant Linuxconf.eu didn't you ?
But then again Cambridge is also a lovely place for a summit ;)