The calendar seems to only be displaying the current month. The 'next' link goes to what looks like the right URL, but the same month is displayed.
Edit: Using the calendar directly is fine, it's just links in the version embedded in the events page which don't work. So anything under http://theforest.org.uk/eventcalendar
is fine, but the links (apart from the events themselves) in it point to URLs of the form http://theforest.org.uk/events?=eventcalendar%2F
<whatever>, which don't change what's embedded.