The first thing that struck me on reading this is;

Is it in the right type of soil!

That is; is the pH right?

The soil should be free draining and have a pH of neutral to acid.

The yellowing could be a form of chlorosis brought on by the plant being unable to pull up nutrients due to perhaps your soil being too alkaline.

Have you checked the pH?

I was starting to wonder if it didn't like the soil, particularly as it seemed to have been happy enough everywhere else it has been. I was thinking about moving it to another part of the garden which has less clay in the soil but is a bit more shady but didn't want to traumatise it. I'll check the soil and then perhaps move it and hope I don't lose it.

Thanks so much for your help.

I'm going to offer a different explanation,because these plants are not at all fussy about the soil they grow in. They prefer the shelter of a wall or fence if in a particularly cold area, but you live in Essex and we're in drought in the south. Unless you watered this plant regularly last autumn after moving it to its new position, and again from end of February this year, because we're in drought, I'm willing to bet the upper growth has declined because of drought. Cut down the stems with the pale leaves, right down to the bottom. The green growth below will grow on up and change colour to yellow once its got enough light on it - feed with a general purpose fertilizer such as Growmore or similar, raked or turned into the soil around the base of the plant.
A photograph would have been useful though, so I could be sure the green growth has just been too shaded rather than it being reverted growth appearing...

It depends whether you are talking about Choisia tenata or 'Sundance'. The ordinary tenata is much beloved by municipal gardens as they grow large, are problem free and make a good backdrop for planting schemes. I think that is 'Sundance' which prefers, free-draining soil, a sheltered, sunny position and suffers badly in cold winds and frosted leaves which causes the leaves to take on a washed-out or brown, scorched appearance.

Watering could be the issue too as I probably wasn't as good at that as I should have been last year. I'm starting to realise that I tend to plant things and not follow up on that, perhaps why I've lost a few plants in the last few years with the dry weather we've had.

I think it could be a Sundance but not quite sure as I bought it a few years ago.

I'll try & get a photo and load it up to see if that helps narrow down the diagnosis.

Thanks for your time & help.

It'll be Sundance if it has yellow leaves, and Jimmytheone makes a good point - this variety is slightly more sensitive to frost damage, but any plant which has been moved or recently planted out will need extra watering.

Another vote for cold wind and frost damage. I have a couple of Choisia Sundance and at some point they've both had this problem. They are tough though and will recover by summer. (I too moved one of them which was pretty mature, plenty of water during summer and it got going again.)

"Should I prune the top, dull leaves off to leave the healthier ones below or will that traumatise it too much?" That's what I did once the risk of frost had past.

Thanks everyone.

I've loaded up a picture but couldn't work out if I could attach it to this thread. It's not a great picture but is it good enough to confirm the diagnosis.

Previously I lived in London, which I'm realising was very sheltered compared with where I now live. It thrived there but perhaps it's just struggling with the harsher conditions.

I've had a look at the picture, but its hard to tell whether the green growth is at the bottom of stems with yellow growth on the top, although it does look as if it might be, in which case its just new growth.
I've just picked up on your comment in your question'I pruned it lightly a few months' back...' Precisely when did you prune it back? Last autumn? During winter?
If you did it last autumn, or during winter, that's not a good idea. The time to prune these is after flowering, round about end of May or early June. This encourages new growth, yet gives that new growth time to harden off before winter sets in, and also won't disrupt next year's flowering. Pruning in autumn or late summer also encourages new growth - which won't be hard enough to suffer a winter successfully.