We'll need some photographs tubby, so we can work through this with you. Please post both a general picture of the mower, and some pics of the brake linkage and the brake itself.
Usually either we find a manual or another member has a similar machine and can help us, but if necessary we can probably work it out from first principles. It may take a while if we have to do that, though.
Did you read the owner manual? It includes a full illustrated parts list, and some warnings about what might happen if you mis-use the brake pedal.
