Thanks guys for the super-quick response. I didn't realise that IFERROR will also return a cell value if it doesn't find a value, I had thought it would only do this if it found an error.
The actual spreadsheet that I have to apply this to has hundreds of rows, when I apply the formula to Column C it will overwrite the existing formulae. Does anyone know of a way to apply this formula to a column of cells such that it will only apply the formula to the constant values but leave the formulae untouched? This might be hoping for too much, I can workaround this by copying the formula sections at a time but would like to know if there's a faster way.