I fitted a new bog standard light fitting and it works but every time I switch bathroom light on the bedroom light comes on and vice versa please help

Did you replace it exactly how it was wired before? I strongly suspect not.

You'd have to tel us exactly how you wired it and what wires are present in the box.

Sounds like you somehow connected the BA/BR feed through one of the switches and it back feeds. Does the new light have a separate switch? Where did you get the hot wire for the light?