Much of the information for this document has been trawled from the various linux newsgroups. I am sorry I did not keep a record of everyone who has indirectly contributed by this route, thank you all very much.
So, to sum up:
Even cheap 3 button Microsoft mice can be made to work.
Configure your copy of X to expect a Mouse Systems mouse.
Hold down the left button at power-on to switch the mouse to MouseSystems mode.
You might need to hold the left button down when starting X.