trying to get blonde roots

Why not just bleach your roots? bleaching and then using a box dye is double processing, so not very good for your hair. Box dyes tend to only be good for hair that is already blonde.

I'm a bit concerned about bleaching, my hair is in good condition but I'm afraid of the hair snapping at the point where root and previously dyed hair meets and the fact I have never used bleach before

You need to apply it carefully to avoid overlap, and use a low volume peroxide. Bleach doesn't automatically equal breakage.

So you wouldn't suggest a bleach bath ans then dye?

My hair is a medium brown I would say so what volume peroxide would you suggest, my mother would be applying the mixture so I'm pretty confident she could avoid over lap, however the roots i dyed last time are a shade or 2 darker than the rest of my hair so I would I be safe putting some bleach on that?

If you want to do it that way (bleach bath and then permanent dye) you would need to leave at least a week between processes.
You could add some bleach to the previously dyed areas towards the end of the processing time. You would need to do careful strand tests to check for lift and timing.

Thanks for your advice I did the bleach and I still have hair and its quite a decent colour!