I don't think yahoo.co.uk supports port 995, so keep it on port 110.
You basically have two choices at this point.
1. a. Uninstall Avira, reboot, then test WM again. If no improvement:
1. b. Uninstall Spyware Doctor, reboot, then test WM again.
If no improvement, proceed to step 2.
2. (You can go here directly, without uninstalling anything.)
Download and install Windows Live Mail (WLM):
http://get.live.com/wlmail/overview
WLM is less prone to suffering bad effects from overly aggressive security
programs.
Either way, email scanning in any antivirus must be disabled, for
reasons explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
Sorry, I'm not familiar with Avira, so I can't give you the exact steps to
disable its email scanning.