Richard,
The carb icing you experienced, whilst affected by the ambient temperature, is mainly caused by the level of humidity in the air. Lower temperatures along with less humidity may well result in no icing. Icing is also more prevalent with engines that are running well - the more efficient the engine is, the greater the vacuum in the manifold (the main cause of the icing). Presumably, you are riding carefully whilst familiarising yourself with the bike - this can also contribute to the icing. As you become more at ease with the bike, you will probably get to be more adventurous in your riding style, and with the use of more throttle, the problem of icing will reduce considerably. As you work the engine harder, the vacuum in the manifold will be less, and the exhaust will be hotter (giving more preheat to the incoming air) - in both cases, reducing the risk of icing up. My 1913 TT 2¾HP suffers just the same - I have found that it pays to warm it up on the stand, then set off fairly briskly, on the basis that larger throttle openings don't generate the ideal conditions for icing.
Don't let the icing put you off riding the bike - the more familiar you become with it's characteristics, the less of a problem it will become, eventually you will be able to predict it occurring, and ride accordingly. 2¾'s are great fun to ride, and can give a good account of themselves in the right hands.
Regards,
Eddie.