Thanks for the response minilite. The 'Log on using clear text authentication' was ticked, but the outgoing server authentication was not. I have now ticked this to use the same info as the incoming mail.
However, since the problem was downloading in-bound mail, I fear the problem may lie elsewhere.
I have also cleared out some files to reduce the overall size of Windows Live Mail - it is now down to 3.13GB. I will endeavour to reduce it further later today.