Apologies mods, I realised the link I posted sent my previous post to moderation. My update download was crawling, took 30 mins to get to 8%. Did a Google on Xbox slow downloads because this isn't the first time. I found the following on the reddit cod forum. Worked for me, after a restart the update finished within 15 mins. I'm on BT infinity and the bandwidth usage monitor was showing above 50mbits. Hopefully this will help some of you guys if you're having slow downloads. Another apologies to the mods for the repost, please nuke the original post.
First off you need to head over to your Xbox One network settings. From there go to 'advanced settings'. Scroll past 'IP Settings' and go to 'DNS Settings'. Change your primary DNS to 8.8.8.8 and secondary to 8.8.4.4. Now go back into your advanced settings and head to 'Alternate MAC address' and hit 'clear'. You'll be prompted to restart your Xbox One and if it worked then you should now be downloading at normal speeds!
I hope this works for anyone else being plagued by 400 KB/s download speeds.