Making progress, but still got a couple of issues, which I will try to explain.
Using Top, I have managed to achieve the calculation I needed (defined as an Expression called "Index") as follows:
((Sum (LineValue)/Sum (Quantity))/(Top(Sum (LineValue))/Top(Sum (Quantity))))-1
But this seems only to work in a Pivot Table.
Perhaps this is because of the Dimensions that I am using?
I have a time Dimension called YearPeriod, and a Drill Down Group comprising Group, Type and Product.
The Pivot Table works if I have the Dimensions in the order "Group Drill Down" and "YearPeriod".
It seems the number of expressions is immaterial, because, as I say, the table does everything I require.
However, if I change properties and make this into a Line Chart the Index Expression fails and the drill down ceases to work also.
Any help gratefully received.