Alf, thanks for the advice but I can't follow it. What does that formula do?

Melvinrobb, likewise I cannot follow the formula in your example, though the numbers are working pretty well.
=SUM(B2:B4)-(SUM(B2:B4)*VLOOKUP(STDEV(B2:B4),$M$2:$N$8,2,TRUE))

Twiggywales..thanks for your advice. I didn't realise it was so easy to find min and max values in Excel. I'll certainly experiment with that!