Thanks, this is the kind of info I need. Will be up on the roof in the morning with tape measure. The installer claims he topped up the glycol and pressure tested the system, so presumably there is no air lock. We have decided to bypass him and get in someone from his HQ.
@Chrisc4290: Cape Town is 33S and therefore your panel inclination is within 9 deg of latitude - more optimal than ours which is 19 deg from latitude.
Some technical feedback for those who replied to my thread.
According to the technician from the distributor's HQ, the optimum angle for thermosyphon is 29 degrees from the horizontal. Above 50 degrees, thermosyphon will not work.
We had the offending panel moved to our main house roof which is 30 degrees, and the water temperature has improved although it is also getting more sun.
Cynically, it turns out that the flat roof installation with 45 deg frame was pushed on us by the installer to make life easier for him.