I've just had a very helpful conversation with the Roche care line.
They have given me a check-list of steps to take to track down where the problem may be:
1. disconnect the tubing from the cannula and then go into the pump priming routine. If insulin comes out of the end of the tubing, it shows that there is problem with the cannula site and not with the pump. Change the cannula and check the end of the old cannula to see if it is kinked or otherwise damaged.
2. if no flow on priming, the first suspect is the tubing - change that.
3. if still no flow, the next suspect is the cartridge - change that.
4. if still getting the alarm - then that indicates a pump problem.
Apparently the occlusion alarm is triggered when the pump detects a change in pressure - upwards or downwards - so in extremes of weather big temperature fluctuations can cause air bubbles to form in the tubing or cartridge which will cause a pressure drop and then the alarm. In my case, I am presently in the tropics and the alarms could have arisen from going from an air conditioned environment to ambient. The same could happen in cold winter conditions. So check for bubbles as well.
It all makes sense to me, so no need to panic!