I have scoured the udf dlls and searched the net, and it looks like I
will have to tackle stdev calculations myself. I'm no guru when it
comes to statistical maths (thanks to that sucking void in my head
where my high-school studies used to be) but it looks do-able thanks
to the existing square root udf. If I'm about to spend a few hours
pursuing a function that can't be done, feel free to warn me of my
folly right about now...

Regards,
Andrew Ferguson