I believe I found the solution – at least it improves things an awful lot – however it requires another 3rd party piece of software to do best (although apparently you can achieve a similar effect using built in effects).
https://vimeo.com/144831669
Here is the link to the video which solved my issue – essentially what you need to do is apply a very specific soft blurring effect to the edge of the sharper parts of the depth pass. Using a regular blur won’t work it has to be offset by a negative vector blur to make it work.