If the rotor arm is loose on the spindle - then yes you need a new rotor arm. However - the spindle itself is free to rotate and is contrained by two little springs attached to two bob-weights down under the fixed plate with the contacts and condenser. This is the mechanical advance mech - and so you should be able to twist the rotor arm forwards (anticlockwise) and it should spring back! If it's not springing back then either the bob weights are seized or the springs have broken or become disconnected. A wee spot of 3 in 1 oil down the centre of the spindle helps from time to time.
I have a 1969 1098 and the distributor was worn badly. It affected the advance mechanism.
I could never stop it pinking, no matter where I set it. Bought a new dizzy and it is absolutely amazing how it has transformed the car. Starts first time pulls all the way through the rev range. No more pinking.
We all rebuild engines, renew this, renew that. But how many times do we renew the dizzy?
Glad to hear your's is ok now Ian - but I still think a 123 would have been better! Did Aldon give you any advance curve information with that yellow dizzy ?
In general -yes - but this 123 seems to give great results because it can be tuned in to suit 16 different set-ups! Would be interested in seeing Aldon's advance curve - or do they keep it 'secret' ?
Edit - And does your ALDON dizzy now contain electronic ignition - or is it still points ?