Any sign of insects on the backs of leaves or anywhere on plant?

Nothing visible to the naked eye. On the backs of some of the leaves just beginning to be affected there is sometimes a scattering of brown dusty particles and on one leaf they appeared to rub off slightly but not on other leaves I tried. Thank you for coming back so quickly!

Looks like a bit of drought, or something's dropped on the leaves at various times - at least at the moment, that's how it appears. Is it planted somewhere shady? Close to a tree, or under a tree?

Certainly not drought! It is in shade and has been in an area subject to starling droppings but there isn't any sign of much bird lime on the leaves. It was looking great until a couple of weeks ago. If it dies I'll have to resort to my old faithful 'Emerald 'n Gold, but I did fancy something a bit taller and narrower. And it will take a few years for a cutting of E 'n G too get very big even if I start it off now.
Serves me right for not having one in reserve.

I find this particular one does better with a bit of sun, but you haven't said what's causing the shade - is it a tree?

If you look on my blog Taking the Plunge last September you can see pics of the bed and the next to last one shows the euonymus. It gets afternoon sun in the summer but none in winter as it is shaded by the shed. It is slightly overhung at a height of about 25 feet by the ends of branches of a very tall sycamore to the East (on the left of the pic but you can't see it)

One of the pics in the blog shows the trunk of the sycamore - the roots on that probably extend a very long way!
Even so, the situation, for this plant, looks pretty good. If it doesn't start to recover as the growing season begins, I'd dig it up and put in a large pot, having inspected the roots first, give it some tlc and when its better, replant.

That's a good idea. The sycamore roots aren't in the top spit so I don't think they will be interfering with it at this stage. I've only got last year's compost at the moment, so when I get some more I'll dig it up and have a look. We have far too many sycamores on the border. At least they are at the top of a five foot bank! Two have gone from that side already and one is keeping us warm as I write. I always worry when I see people leaving young saplings to grow where they shouldn't. Thank you for your interest.