I suspect the pot isn't big enough and there hasn't been enough water. Despite the rain the leaves will have prevented the pot get fully damp. If you can pop it in the ground and water well for the next 2 weeks. By then it will have put out new roots so feed it them.

There are possibly many reaseasons for the condition. The winter has been extra cold and subjects in containers could be suffering from the roots becoming frozen. The frost could have burned the buds and the rains can easily wash away the nutrients in the container. If your garden soil is neutral or acidic, they I would consider replanting in the ground.

Thank you for your helpful replies.

You have set us thinking that our garden needs some fairly drastic reorganisation - in some areas at least. We plan to remove a couple of dead or dying Griselinias, part of a hedge and that were affected by frost, and replace them with a few unhappy things we have in pots. We also will make more space for planting things from pots (notably the Camellia) by digging up part of a grassy area that currently serves little purpose and gives me extra work by having to mow it occasionally. Also, some other shrubs that are not in pots are not ideally situated, so they will be moved too.

Sounds like I've got my work cut out for a bit. Thanks again!