He is to take a period of convalescent leave and is not expected to return to PR.
Shareholder representative and non-exec Jeremy Swan has stepped in as acting MD while the agency searches for a successor to Lock.
The firm, which works in corporate reputation management, PA and litigation PR, has also appointed a part-time accountant to replace finance director Sunil Sharma, who has resigned from the agency.
Director Richard Elsen, who said it was 'business as usual' for the agency, added that a replacement for Lock would be appointed 'between now and the summer'.
Plans to expand the agency's work overseas remain on the agenda. The agency currently operates in London and Brussels, serving clients including Direct Line, Elite Model Management and Tommy Hilfiger.