Ok, what I'm after is the formula for calculating the % variance
between a current value and the previous value.
I think this should be (where a is current, b is previous) ((a-b)/a)*100
I may be wrong about that - can someone confirm what it is AND provide
a link to a reputable maths source showing that.

Thanks

Hello charltonian-ga,

Thank-you for your question.

I believe the formula you are looking for is usually called the
percentage difference, rather than the percentage variance which may
be why you have trouble locating your answer.

The formula you have stated is the most common way in which you can
calculate the percentage difference, the other is: ((a/b)*100%)-100

The two different formulae are discussed here:
http://www.mathsisfun.com/percentage-difference.html

Further evidence of the formulae can be found here:
http://mathcentral.uregina.ca/QQ/database/QQ.09.01/tedd1.html
http://mathforum.org/library/drmath/view/58083.html
http://www.gcse.com/maths/percentage_difference.htm
http://www.mdx.ac.uk/WWW/STUDY/glonumar.htm
http://www.cliffsnotes.com/WileyCDA/Section/id-110141.html#math16
http://www.gomath.com/Questions/question.php?question=46424

I hope this fully answers your question.  If you require any further
assistance please ask for clarification and I will do my best to
respond.

Excellent. Simple enough in the end but I needed it!