Oh Dear, it doesn't look a happy bunny, does it? I thought we might have had a case of droplets of water on the leaves causing sunlight scorch, but those yellow leaf margins speak of something else. To be honest, I don't really know. Sorry!

I don't think the container you have your Pieris in is big enough... And the soil looks very dry.

Well, thank heavens you posted a photograph. The answer is very simple - the Pieris is in a shallow trough. Get it out of there and put it in a deeper, ordinary pot, preferably big enough to accommodate any roots its currently got - you'll need to turn it out to see what shape they are and how extensive, then repot in ericaceous compost in a proper pot, not a shallow anything at all. And if it gets a lot of sun, keep it well watered. I don't know which variety of Pieris this is - it looks like one of the P. japonica ones (which don't mind sun) rather than P. forrestii, but the latter varieties do not like too much sun.
A note about troughs and window boxes - they're not fit for growing anything other than temporary bedding or dwarf plants.

"A note about troughs and window boxes - they're not fit for growing anything other than temporary bedding or dwarf plants."

Totally agree Bamboo!

Thanks very, very much everyone. Sorry i seem so naive! G O Y at it's very best! You never stop learning. The plant has obviously out-grown the container. I shall follow your advice Bamboo and re-pot asap! Although i have watered the plant copiously, now i think about it, it does rapidly dry out. The healthier plant is in more shade but i will also re-pot that as well.

The problem is, it can't put down deep roots in that shallow container, so its obliged to push everything sideways, near the surface, where it dries out rapidly. Which is why I made the remark about troughs and window boxes generally...

Just came in from the garden Bamboo, you are absolutely right! Have now put it into the biggest pot i could find. The roots were so tight when i turned it out, i could hear a sigh of relief now it's in it's new home which is about 3 times as deep and twice as wide! Thanks again, Tiger.

There, you see! There's always someone on here who knows these things. Am now (rather shamefacely) making hurried plans to move all my shrubs, etc., out of their pots and into proper beds.

They're alright in pots, Gattina, well, depending on what the shrub is and how long its been in the same pot - the problem here was the TROUGH!

I must say Gattina, i'm glad i did it. I will post some pictures of the plants in their new homes. I had one giant pot that i put the sick plant into and my neighbour came to the rescue with a pot for the healthy one. Her pot was not quite what i had in mind but now it's done, i think it's not too bad! What does everyone think?

I will send new photo's as soon as i have sent this message.

I am getting OH to make me some largeish wooden troughs for my annual displays, very like the ones that Willinilli made so beautifully, and the narrow plastic troughs have been consigned to the greenhouse, planted up with vegetable seeds (beans (3 types), squashes, courgettes, etc., so they have a head start once the weather decides what it's going to do. I have used lengths of plastic guttering to get seeds like parsnips and radishes started, and these will be laid into the prepared beds once it's had enough rain to soften the soil to the point where I can actually get a fork into the d****d stuff. Has anyone else tried this method? I've never done this before, but I can't think of any reason why it shouldn't work.