Just stumbled over this:
http://austin-rover.co.uk/downloads/drive_06.pdf
It would seem that pre late 2001 Symphony equipped cars had the same diversity amp as the Hi Line system, namely the one depicted in Keith's photo and then project drive killed it. My car is a 1999 model so it stands to reason that I do not need a new aerial amp.
Subsequent question: I am a lazy ****** so can pretty please someone tell me what kind of connector is used for the main antenna cable (the one with the yellow thing on it in Keits pic above)? BNC? Regular TV style? Something else?
Bonus points will be awarded for telling me the impedance for the diversity cable. The main antenna cable I assume is 50 ohm.