....it worked! I stripped the Mac Pro today (finally - how long have I kept you waiting) and installed the two Quad-Core Xeons in place of the Dual Core ones.
YouTube tutorials played on the Macbook Pro as I worked. One part after another was removed, dusted and put aside.
Then came the new CPU's. I bought them months ago and never tested them to see that they were working. While cleaning one, I found that it had a stick on adaptor to make it fit a standard socket! Disaster if I fitted it like that. Quick removal of adaptor and a hope for the best.
Arctic Silver applied. Heatsinks reattached. Parts put back in - not without difficulty. Power applied. Chime!!! Video. 10.7 loads and tells me there are now 8 cores! Yipppeeee! Fans run normally.
Success. Photos on the site soon.