Craft Commerce: Sage Pay - Redirect Required after payment?
Looks like your redirect does not contain the full url back to your site. 

The error shows:

195.170.169.29 - - [20/Jul/2016:09:05:17 +0100] "POST /index.php/actions/commerce/payments/...


But I would expect to see a url starting with http://sitename.com/ since your gateway needs the full url to redirect you back to your site.

Make sure you have a siteUrl set up in your craft settings: https://craftcms.com/support/site-url