Excel for Mac 2011 - autosum function not working

Hi - I am running OSX Yosemite 10.10.5 and have Office for Mac 2011. In Excel, the autosum function seem to have stopped working - any ideas?

I've done all the usual including:

- making sure that the cells are formulated as 'numbers'

- not copying from an external source (so I know they are numbers, not 'text)

- tried using a table within excel

- make sure that the excel preferences>calculation ... is set to automatically

- made sure that the region is UK (as this affects how data is sometimes displayed)

- everything else I've found via searching the internet, including 'cleaning' my data, copying it into word and clearing any formatting then pasting it back again.

I'm stumped - anyone else had this problem? Below are 4 images to show what happens:

If I manually type in the formula, I just get 0's:

If I try the autosum function after selecting the range of cells, it just inserts =sum() in the first highlighted cell (see below)

If I select a cell at the end and click autosum, same problem:

And if I try and do it as a table, same again: