I'm sure I am being very very dull, but I can't seem to set an R1C1 style address via code

I've got the following

VB Code:
  1. tmpAddress = "R" & StoreRow & "C" & StoreCol

The value of tmpAddress is currently reading as "R7C1", and is defined as a string. Hmmm ... is THAT my problem .... defining it as a string ?

I'm trying to use it thus :

VB Code:
  1. objexcel.Range(tmpAddress) = iGrid.CellText(iRow, 15)

but I'm getting "Method 'Range' of object'_Application' failed" errors

Can someone point out the error of my ways please.