Here I am again, still looking like a numpty! But I'll leave all this here anyway, in case other numpties have the same problems. Have upgraded to .NEW Framework 4.5 which solved the problem. But now I find that the spare computer we were trying this on is only 32-bit Windows 7 and it needs to be 64-bit. Sigh.
Time to get out into the spring evening fresh air though before it's too late, so this will have to wait a bit longer. Thanks anyway @cryptos for your excellent instructions, and maybe we'll be mining by tomorrow!