Martin,
When i examined the system, i assumed it was a spring pushing it back in position. This video describes the system as used in the SE/puma engine.
http://www.youtube.com/watch?v=Ba-oyIjGWGE
Ford's TIS manual describes the ST170 control method as identical to the Puma version. I'm not sure this is the case. If it isn't, and it doesn't use advance/retard chambers, but just a spring, then that would be far easier to control.
Lots of testing to come when mine finally fires up.