Just thought I would update this thread -
Thanks Sam for help and advice made a lot of sense
This morning I plugged in my iphone via a phone > RCA plug adapter - Buzzing noise was gone. This led me to think problems with wiring fro back of HU to the amp - it picking up interference on the way
So I pulled out my HU, re routed the wiring loom under the passenger footwell rather than under the glove box - buzzing noise 85% gone. Then I re routed my earth wire as to makkans post above and did a bit of tidying up with wires. As a result 90% buzzing noise gone. I had a spare ground loop isolater (yes I know some of you will be shaking your heads at me) I decided to plug that in and buzzing noise 95% gone. You can still hear a very faint buzzing noise when on mute but not when volume on whereas before the buzzing noise was constant no matter what volume.
Thanks to everyone really appreciate your help
I will be typing up a mini how to if anyone is interested as I took some pictures along the way