Further development. I removed the CPU to check the socket pins again and this time went over them very carefully with a magnifying glass and found one pin twisted 90° anticlockwise, crossing the path of another pin, it definitely would have made contact with the other pin when the CPU was installed.
I tried to set it straight with technical tweezers and succeeded as far as I could tell but after replacing the CPU the mobo & CPU remained unresponsive. I cleared CMOS again but it did not not help. I removed the CPU again and all the pins appeared to be straight, but its difficult to know how they respond under pressure if any of them is twisted.
At this point I do not know if the pin was like that when I got the mobo. I remember finding it difficult to get the CPU in cleanly and seated evenly so it may have been the extra thickness and I dont know if it was my own doing, I honestly don't know but it did go in a bit "corner first" kind of thing! I did inspect the pins before adding the CPU on the first occasion but only cursorily and did not really know what I was looking for as I have never built with this type of socket before. I don't think I would have spotted the crossed pins as it looked like part of the pattern when I finlly found them.
I also don't know for sure if this has been the cause of the malfunction but suspect it was and I dont know if any permanent damage has been done to the CPU (or the mobo) by trying to boot like that. I don't have another Socket1150 PC to experiment with and find out. The only good news is that I had the whole order insured for installation damage, so I am not going to be out of pocket, but its very frustrating to have such a nice PC not working
and RMAs are such a pain.
This is my first build with Socket 1150 and in my opinion LGA is not as robust as PGA.
EDIT Now have a RMA for the mobo.