Under international law the first entitlement of a signatory to a treaty, in the event of another party's non-compliance, would be to consider the treaty null and void. This would be of little practical value to us in the event of a breach post 1997, as the Chinese would have resumed sovereignty.
A second option would be to take a case to the International Court of Justice. But China has not accepted the compulsory jurisdiction of the International Court of Justice. It would therefore not have jurisdiction in a case involving China.
In theory it would be open to us to complain to the Security Council if we considered the breach might endanger international security. But the Chinese would doubtless use their veto in such a case, the exercise would be valueless.
We would be able to put a motion to the General Assembly of the UN but, even if a motion critical of China were to pass, the Chinese would be able to ignore it as many other countries do.