Ah ok, I was following this guide on the mtbr forums http://forums.mtbr.com/shocks-suspension/switching-poploc-knob-rs-revelation-436769.html that instructs you to drill the rivets and remove the spring. I have the correct sized rivets so re-assembling is straight forward.
So I could have just unscrewed the brass top cap (L/H thread so clockwise to unscrew) via the hex knob that the adjuster lever is housed on. Then removed the return spring, fit the detent bearing and spring in the black outer top cap and refit the push rod. Makes sense.
As usual Loco an amazing source of help. The rear shock needs a service soon, it will definitely be coming your way.