Thank you John.
I apologise for the delay in acknowledging your input - a short but warm break in the ME kept me away from the internet.
It does make more sense now. I will try different CIs during the flight preparation phase to see what sort of 'scatter' I get which might help to explain further.
Again, many thanks to you and your pal for the time and effort.
mcdhu
PS Just finished Tony Blackmaman's 'Vulcan Test Pilot' - an interesting read.