Morning! Had a lovely sleep thanks.
Right, I suggest Clairol Nice n Easy 118B Natural Medium Caramel Brown for your roots. Its quite a copper brown shade - not mahogany, but enough brown in it to help match up, and enough warmth in it (hopefully, its a very warm shade) to blend with the body of your hair. I know the roots are already warm but lack depth.
If you do that and it still doesn't match, i.e not enough depth or not red enough, you could try the 120B Natural Dark Caramel brown, but that's quite a dark shade. Start lighter, you can always add "more colour" later but you canne taketh away!
Whilst a strand test is not an option, you could do a "patch test" on your roots - mix a teaspoon out of each bottle in the box together, and choose an inconspicuous spot to the side, halfway between your ear and the top of your head, paint on the dye to your roots only (do it one morning half an hour before you were going to wash your hair anyway), and once your hair is dry, find the spot and observe the colour match. If its good, then you can mix up the rest of the colour and do your roots only - I suggest you buy a cheapie foundation brush and use that as you don't want to be dying over the rest of your hair if possible - you will get colour build up and overprocess it.
If the patch test is all wrong, we can think again!