Had a callback this afternoon from AppleCare UK to say they're starting to get more reports of it, seems like it's definitely an Apple mistake somewhere along the line, so rest assured if you can't get it working, it's not a fault with your device or the way you've installed the update. I'm expecting another update from them late tomorrow night UK time so will keep you all posted.
In the meantime I'm trying to find out through you guys & word of mouth if this is an issue that is affecting all users of specific devices or is just completely at random. I know a lot of people who are having this issue are using an iPhone 5S, but I've not yet had someone say 'oh it worked fine on my 5S'. I am also having the issue on my iPad Air and I am aware that some users are having this issue on the iPhone 6 too. The engineering department at Apple are investigating what's gone wrong though, so I'm sure they'll have an answer soon. If any of you can offer a bit more information concerning devices affected, and if you know of anyone who has had success in getting the full update on a device others have had this issue with, it would be appreciated As it will probably help find the root cause faster.