So true, Lee-Roy but I was going for the other angle, that this kind of distress can happen from over watering, rather than too little water. London UK is usually fairly far from a hot and sunny climate.
Sharkfur, have you checked on the drainage in your nice planters? Maybe dig an exploratory hole or two with a long narrow trowel, and see if the soil is soggy, or dry down near the bottom of the boxes. You can do this between the plants or near the corners where you won't disturb the roots too much. Wiggle your finger down in there and feel the soil.
On the other hand, the planters are a dark color, I wonder if they are getting really hot during the day when the sun is shining on them? That sure would tend to dry them out faster.
