It is an offence to fail to comply with an Enforcement Notice, once the time limit has been reached and no appeal is outstanding. A person found guilty of an offence is liable to an unlimited fine, and the courts will take account of any financial benefit stemming from the offence. You should immediately obtain professional advice.
The local council does not have to take enforcement action, as it is a discretionary power. An enforcement notice should only be issued if it is in the public interest to do so, and if it is the most expedient way to resolve the breach of planning control.
Consult the Government website for further guidance on enforcement and post-permission matters.