As I've arranged the D7 to have its annual MoT on 17 June I thought it best to replace the spring ASAP. But, after removing the side cover - then the clutch drum - I discovered the spring to be intact. I think the lever must have moved inwards so far and was catching on the side cover.
Checking the spring, there's plenty of springyness left in it, so it will be OK to reuse.
As you can see by the 'graceful' curvature, the chain is well and truly knackered! But a new one is on the way, courtesy of http://www.rexcauntracing.com/ (I have paid for it though!)
Hopefully, by the middle of next week Little Willie will be up and running and a lot quieter than before! Or at least give me some new sounds to investigate!