Thanks. That is brilliant.
I now feed in the "Order" of the cash movements within each investment: 1, 2, 3 etc
This generates the offsets that you need for your formula = 1 - Order = 0, -1, -2, etc
For some unknown reason this also fixed a problem I was having with XIRR
XIRR had been refusing to give negative internal rates of return. Now it is fine.
Thanks Problem solved.