iPhone video app that lets you switch from front to back (in) camera mid recording? Does anyone know of any iPhone (6) camera apps that lets you flip from the front camera to back camera whilst recording a video please? 

The standard camera does not, and I can't seem to find any apps that do! I don't mind paying for the app
There is an app called MoviePro. It has lots of video options. It allows you to switch between front and back cameras.

Find the app here : https://itunes.apple.com/us/app/moviepro-video-recorder-limitless/id547101144?mt=8