Just a quick update. It looks like everything is now fine:
I suspect that a combination of things was causing the BSODs, although of course the memory management was the main problem. The BIOS change didn't solve the problem immediately, but after clean-reinstalling Win 7 and reinstalling the programmes I need only when I need them, and then following up some errors reported in the event viewer (and googling for solutions), it may well be that I now have the most stable system I've ever had on this machine. (And of course I've made an image!)
More importantly, I also learnt how to re-seat the memory chips (at last!, I was so worried about touching them!), and this could well also have helped.
So many thanks to you all

for your assistance.
Of course, I could be tempting fate by writing this, but if anything goes wrong the manufacturer can still send me replacement chips at a later stage.
Thanks to all :-)
Kind regards
David