Quite simple: I undid the cap nut on the top of the threaded pillar so as to carefully remove the router body from the base. The plunge was a little jerky so I assumed some dirt or debris had somehow found it's way into the spring tubes. I wanted to check; clean and re-assemble the whole shebang. However, the springs were stronger than I thought and I almost ended up chewing the top of the router body as it shot upwards. I gathered up the springs and the plastic spring liner; cleaned and greased the springs and re-assembled it correctly: or so I thought. The rest is history - and so is this router if I can't find a way to fix it. Downloaded PDF explains nothing. See; I told you I'd been stupid!