OK, select the column of cells you are trying to sum. Select -Replace- then "find what" and type the currency symbol (in my case) £ in the 'Replace with' leave empty. then press button 'replace all'

With the column of cells still selected insert the currency symbol from the drop down menu and it will be ok. Essentially even though it shows a currency symbol it's actually text and excel is not recognising the symbol so you have to delete the old one and insert a new one from the correct menu. This is the problem I had when importing from a csv file - solved it straight away.