Quite a slide in just five years!
It is blamed by the incoming VC on general drops in student numbers in the UK, that most notably affect part-time students (like AU, all of the OU’s students are part-time). A recent article suggested a 37% drop in recent years in the UK HE sector overall, so 28% is perhaps not that awful, in relative terms.
The drop is not too surprising, given that UK fees have risen precipitously in recent years thanks to decades of attack on higher education by both labour and conservative governments. In the OU’s case, according to one of the commenters, this equates to an increase from £500 (a bit over $900 Canadian) to £1600 (about $3000) for a course. When course costs rise, part-timers (many of whom are self-funding and were, in the past, doing it for love as often as for career change or advancement) are inevitably going to be the first casualties.