OK that's helpful, thank you. The clerk to my council does the same and indeed sometimes works from the parish office and sometimes from home. The downside is that it can be difficult to know when the clerk is at work but we provide a mobile phone and she's happy to be contacted at most (reasonable!) times and will say if she's not working that day. On the positive, she is generally available to deal with whatever we throw at her, is happy to work whenever there's a need so we don't have to fit meetings etc. around her and we don't need to pay overtime as she will adjust her working week to fit the pattern that suits the job rather than her prescribed hours. We don't ask for a weekly timesheet and I'm certain some weeks she works well in excess of her 30 hours and some weeks she'll work much less but over the year it should even out.
We have a staffing committee (of which I'm a member) which meets quarterly. If you have something similar, you could ask for her timesheets at that meeting? Be prepared, however, for the clerk to request overtime or additional hours if it shows she's regularly working over her contracted hours.