The short answer is possibly yes. The question was from a reader but on reading the question it is suggestive that something has taken place outside of the work place.
The test for harassment is in the eye of the recipient even if you felt you had not intended the effect to be this. The issue is whether your ex-partner felt that the conduct was unwanted and had the purpose or the effect of violating his/her dignity or more likely in this case creating an intimidatory, hostile, degrading, humiliating or offensive environment for him/her. This definition comes from s26 of the Equality Act 2010. The conduct does not need to take place in the workplace particularly if the conduct impacts upon the workplace.
Whilst the question is not detailed enough to provide a thorough answer and does not constitute legal advice hopefully this will give you some information. It is better if possible to avoid your ex-partner day to day and especially at work.