Hi, Thanks for all your input. You have restored some faith in my system and it's supplier. There are certainly a number of factors involved. Temperature effect is certainly one, the other may well be losses because of atmoshpheric pollution or just overall absorbsion. I note even the lightest of cloud cover causes a drop in output. BPO's experience at higher altitude in NZ seems to support this.
With regard to temperature, the specs. on my panels show an efficiency voltage loss of 0.35% per degree. My bank of 10 panels shows a voltage drop-off of about 45V (12% down from 375V) from 8am to midday,and the current rises from 1.5A to 5A in the same time. At 0.35% per degree, the panels must be at about 35 degrees above ambient to record such a voltage loss, which I guess, is possible. Perhaps I need to put water-cooling on the panels.
Wouldn't it be nice if companies told prospective customers what they should expect from their systems?
Robert