Barmaid is right. A chimney breast attached to a party wall, which this must be, is covered by the Party Wall Act 1996 if an owner wishes to remove it.
You can try for advice from Building Control but they do not legislate this work. They may be interested, though, on the grounds that the integrity of the party wall could be compromised. What is supposed to happen before the work occurs is that the neighbour should serve notice on you under the Party Wall Act. That entitles you to appoint a surveyor, who agrees on your behalf with another surveyor working on behalf of the neighbour to construct a party wall agreement. This agreement assures the structural integrity of what he is doing. THE Person wanting to do the work HAS TO PAY THE COST of both sets of surveyors.
Try phoning a surveyor in the morning. Such people do a lot of these sort of agreements. What I'm not sure of is what happens when a householder has failed to follow this process and started the work.