Thanks - that worked a treat - I just pushed the signal arm (sort of gently) up and it re-engaged - works a treat now - well sort of!
To explain the 'sort of' for the curious...
This is an LMS style N gauge home signal on an LNER wartime layout..
Push the button and the arm now goes from horizontal to up but the motor keeps making a noise.
A quick tickle of the button and it stops making a noise.
The arm then stays up until the button is pressed a third time and down it comes - (just like the real thing...)
So it is now functioning, but in its own idiosyncratic way...
As long as it keeps functioning, that is just dandy...