CCL did confirm by phone that the first GPU was faulty. However, the problem appears to be solved. The new card worked in my son's PC so it had to be PSU or motherboard. We did a partial dismantle then removed and did a continuity test on the two PCIE power leads plus a no-load volts test on the PSU output sockets. All was OK. Reassembled, tried again and it worked!
Unless there is an intermittent fault somewhere, the only logical cause must be that we were not plugging the "+2" part of the 8 way power connector into the GPU properly, although son and I had both done it two or three times each. The "6-way" parts latched into place properly, but the "2-way" parts having been detached don't then have a latch and maybe we had not been fully inserting it.
Anyway, my thanks to those who responded. Lesson learned - always check the connectors are properly inserted, then double check.
I can try a bit of simulated flying now!
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