So, after some testing the clutch spring isn't giving anywhere near as much assistance as I need.
I know why. It's to do with the length of the lever arm on the ring welded to the pedal. On the VW Golf pedal it's a good 30mm longer but my issue was space was limited and the amount force the spring would exert was always going to be unknown until I built and tried it.
I could start again with a redesign, and I'm fairly sure I could get it to work, but I think I'll go back to using a remote servo on the clutch line. It worked really well last time and the only reason I didn't want to use it again was because I wasn't happy with it mounted under the wing. However, I've done a trial fit and I think if I'm careful with the positioning I can get it mounted in the Clubman's engine bay and get it to clear everything.
Bad luck mate, at least you gave it a go. Why bother with a servo anyway?