Hi,
I've managed to establish which software was causing the issue, it seems to be McAfee BT Virus protect (free security software with my BT Broadband).
I've found that if I uninstall the McAfee software then issue disappears but if I reinstall it the issue comes straight back again. I'm just posting this here for reference in case any one else has the same issue.
I've tried to contact McAfee support but currently their website returns an error when you try to raise a ticket. If I do manage to get any feedback from them I'll add to this post.
Thanks.