Initially I reported the issue to Garmin UK via their online form.
Their reply has just been, reinstall it and if it's still broken contact your dealer.
Garmin's software but they're offloading the problem rather than wanting to get involved. It was the same a few years ago when they broke their TPEG digital traffic system. They weren't initially interested until they presumably got hundreds of reports about no traffic on standalone units.
Fortunately this issue is in hand via another route.