Best hair dye for resistant greys?

[pinktortoise]

This is really depressing me. Have been using the root touch up stuff (nice and easy?) but it is not covering the grey for more than two weeks. Am off on holiday soon for 2 weeks and don't want grey roots.
My hair is dark brown and I only do the roots.
Any success with any particular brands?

I have quite a lot of grey, I daren't think how much! My natural colour is medium - dark brown and I have just started using Superdrugs own permanent hair colour in Medium Brown. I always just do the roots maybe every 5-6 weeks as my hair grows quite quickly and it works really well. And only £3 something. I pretty much use the whole lot just to cover my roots. I hate hate hate having greys!

Belle colour, natural coverage i am an expert.

So superdrug or belle colour to try..thank you. Pirate cat you are an expert so have you had trouble with getting a dye that will actually stay on pesky grey and what leads you to belle colour?