Write to him asking him to confirm details of the Complaints Handling Procedure - Tell him if you don't receive those details within 14 days you will go direct to the Law Society. The Complaints Handling Procedure will probably involve writing to the Senior or another Partner within the firm, but you need to make sure you confirm details of the basis of your complaint. Sounds to me like he didn't really know what he was doing - the Law Society should really be told about professionals acting in such a manner, so you could always report your concerns to them.
Best of luck