Ok I checked the back of the voltage regulator on the car - definitely a standard unit not for electronic, based on what RR said and the article that TJP posted.

So I bought the electronic one in the link above and installed it this morning. I think it did the trick. Ammeter in car reading near dead center, and no erratic movements.

New readings :

Battery at rest : 12.71v
Battery at idle : 14.7v
Alt at idle : 14.9v
Voltage Reg at idle : 14.1v

@ 1600 or so RPM readings didn't change much across the board.

I took the car for a short ride and no ammeter jumps and read close to dead center.
Obviously I'll have to go for a longer ride, but bad weather heading in this afternoon, so that'll have to wait ( hopefully not till Spring! )

In the meantime, thanks for all the input, as always