So, I went into "simplify" in "scene properties" and added 1 AO bounce. I went from 15 minutes a frame to 47 seconds with NO grain! The catch?
An AO bounce, takes light from the world background (I think) and adds this into the scene (interior or not) It also, removes all reflections from glossy or metallic surfaces and replaces them with the world background probe/map.
This Didn't help, considering my whole animation was based around a showcase for a product that you use on a MIRROR.
Go into the "world Properties" enable "AO" and increase the Distance until these reflections disappear. Then back to scene and increase the light bounces for AO, until your scene looks right.
You will find your scene is a lot more exposed, however, if you use the "filmic" colour management, and set it to one of the higher contrasts, and then render in a HDR format, you can reduce the exposure and keep the correct contrast ratio.
Now, I didn't stay at 47 second render time per frame, however I did cut if from 15 minutes, to 4 minutes. Which, is a massive difference!
I hope this helps anyone with a similar issue. Please remember, this is only useful for indoor scenes! an outdoor scene won't have the same noise issues, and fiddling with the AO, will massively increase the world background becoming oversaturated.
If you couple this hint with the Denoising feature, you will get a really good looking render, for a third of the time.