I think this is a difficult one as jointly owned hedges are normally quite unusual as they normally planted on one side of the boundary or another. I suspect that if they're not gardeners they really can't (or don't want to be bothered) with coping with the regular maintenance that a conifer hedge involves. So this really does put the impetus back on you and I'd suggest talking to them fairly soon and offering to get their side sorted out, although this will obviously cause you effort and inconvenience. By offering you will then hopefully get some feedback as to how they feel about the hedge. I fear that if their side grows another foot it will reach the 11 foot legal limit and then you could end up having real problems about how the issue is resolved, so it's best to take action fairly quickly. But if they allow you to trim their hedge once, you will probably have a job for life as they will undoubtedly expect you to continue doing it.