In the end Bergey & SIAC kindly solved the problems I encountered with our Bergey Excel 10kW turbine by replacing the turbine itself. Obviously the tower & wiring remain unchanged. This has revolved the issues and we also benefit from the lower start-up and increased output of the updated model. We are also now using Bergey's Powersync II inverter, which performs well. Unfortunately, no one remembered to say that we need to uprate the fuses in the tower disconnect box from 35 amp to 45 amp to take account of the increased output, which I discovered the hard way when 1 phase 'blew' a fuse.
Jon & Ian from SIAC did a great job supported by the guys from V3 Power.
I expect to obtain circa 19,000 - 20,000 kWH p.a. from the turbine now, compared to the 13,000 p.a. or so I have typically been getting for the past 7 years. I will post an update here in due course!
More details of my new Bergey turbine can be found here.
The is also info here.