Stopping adjacent neighbour on our shared wall

We live in a very small terraced house in North West London built in 1936. The house adjoining ours next door with a shared dividing wall to us has been up for sale for a while and recently someone arrived and said in a not too friendly way, " prepare for some banging and we are taking away the fireplaces". I assume he might be a developer since he didn't show any interest about anything else that we might have been able to tell him, if he wanted to live here himself. The House walls construction are of single brick thickness and the fireplace vertical as it extends into the rooms on ground floor and first floor extend by 14"(36cm) by 47" (120cm) in both front and back rooms, upstairs and downstairs, on this shared dividing wall. The houses are not built to very good standards judging by the quality of the wooden floors and roofing timbers. Also the cement seems rather crumbly, as if it has too much sand in it. Now what if he suddenly starts knocking away the bricks. We do worry since we don't know how structurally crucial this might be. Talking to another neighbour she says that she thinks he needs permission from us to do this work. But what if he just goes ahead with it anyway? She also mentions an 'injunction' to stop him, but we don't know how to go about such a thing, or whether this is the best way to go? Grateful for any advice. Thanks.