After pursuing every possible Outlook issue I was convinced that the problem was elsewhere and probably with the server. It took a lot of perseverence with a number of BT Yahoo advisors but I eventually ended up with a tier 1 advisor who accepted that the problem was with their server. Not only that, but it was a known issue!

BT Yahoo are splitting up and they are experiencing problems with migration between the 2 servers while BT take over the e-mails.

Pleased that the problem will be solved shortly but furious that BT Yahoo haven't informed their customers of a known issue.