So I will have two red wires in a block, the two black wires to neutral and live on the new fitting and all earths into earth. Where would you put the blue and brown wires from the attic light? Are you saying the brown live wire can go with the Reds? What about the neutral- attached to the neutral on the new fitting ?
Yup, brown with the reds and blue with the neutrals.
However, you aren't going to know which is your neutral and which is your switched live without a tester.
You can connect a basic lamp to the reds, and then try it in each of the blacks you have - the one it works in will be the neutral. Good idea to mark the other black somehow (ideally sleeving) so that you know it's the switched live.