Sorry everyone, it's one of the symptoms of the server migration thing we did last weekend. Apparently some piece of third party software - as in 'that's the third party the tech team have been to this week' - is to blame.
We're aware of the problem and the techies are rocking round the clock to sort it out. Or maybe they're just at another party. In any case, it should be sorted sometime this year. Hopefully.
In the mean time, the new latest replies option makes it easier to catch up with thread updates etc.