Glad I had a look in the PSU, and an excellent bit of remote diagnosis guys - the smoothing cap is not looking healthy
It is only a few weeks since I last looked in the box, and it happened since then, so hopefully no damage done.
Does anyone know a UK source of good quality (name brand, not necessarily audiophile) capacitors? I was thinking of Farnell with Vishay, or hificollective with e.g. Nichicon. I guess I will replace all the electrolytic caps whilst I am at it.