Wow I found a zombie thread on mumsnet (didn't want to reanimate it!) where someone, I think Tattydevine, advised pre softening hIr with hydrogen peroxide before dyeing your hair at home.
I have got so bloody annoyed with all these home hair dyes with '100%' grey coverage blah blah and I would take my time applying it, rinse it off and I could still see glimmers of grey at my temples.
I tried ore softening this evening and I have complete perfect coverage and now I don't have to spend a fortune at the hairdressers every month.
Thought I would share the tip, and thanks mumsnet!!