As it looks like no one will be looking at this again until Monday I have tried a few other things, the results of which may be useful:
1. I tried turning off Behaviour Shield. Interestingly this made no difference to the CPU usage for the process UNTIL I restarted the laptop.
2. After restart, CPU usage was down to around 0.5% - very acceptable.
3. I turned Behaviour shield back on and CPU usage did not change. Concerned that the turn on would not take effect until I restarted, I did that.
4. After restart CPU was still under 1%. I selectively started other Apps - Chrome, Windows Media Player and File Manager are the three I use most. None of these had any effect on the CPU usage of Behaviour Shield.
So at present I am back down to sensible levels - I will report again if this changes.