can I add extra bleach and/or developer to shop box dye?

I wouldn't as you have no way of knowing what is already in the box dye or what developer they're using. If you have bleach powder and developer could you not just do your regrowth with that to the same level as the rest of your hair and then tone instead? Without seeing your hair though I'm not sure, I just know I'd be really reluctant to add extra developer or bleach to a solution I wasn't sure of. Glad to hear your hair is finally starting to work out though, it's great what that happens 🙂

Thanks, yes it's not too bad  😉  Yes I could just do the roots with the bleach and developer, what strength developer would I need?  I'd say my hair is naturally light mousey brown, although typically the roots against the blonde look much darker than that!  when you say tone what do I need to look for?  semi permanent box dye?  I went into Sallys back along and asked for an ash toner, they suggested the blue shampoo!  or could I then just use my permanent dye on top - or would that be too damaging?

I wouldn't bleach your hair and then use another permanent colour straight away, as there is also peroxide in the permanent in levels you can't be sure of.
Toners are typically purpley, loads of people have success with various shampoos, I think there's a thread on it somewhere.

As for the volume of developer you'd need, I would say a 20 vol but having not seen your hair and how it takes that's only a wild guess. Definitely do not put the 40vol on your scalp, I know some of them to claim to be safe for on-scalp use but I'd be really wary. You can get some really nasty burns from that stuff.

You might be able to get away with a bleach bath on your roots instead. Then the box dye will lift it the rest of the way. Just wait a day or two between bleach bathing and adding the dye. The roots will develop quite quickly because of the heat from your head and you don't want the roots to go too light or else it won't match the rest of your hair.