We have a woman worker who gets a lot of fairly good-natured ribbing because she has short hair and wears trousers, though she is not in fact gay. Do we have to put a stop to the jokes?

Yes. Whether she is gay or merely perceived to be gay is irrelevant under the new legislation: what matters is the fact that she is the butt of jokes because of other people's beliefs about her sexual orientation. The fact that the jokes appear to be good-natured is also irrelevant. The test is not the intention of the people making the jokes, but whether these jokes could reasonably be considered to have caused offence. The response of the victim is also relevant.