You're going to have to help us help you, lynn. This is sort of like a patient going to a doctor saying, "I've got a headache. What's wrong with me and what are you going to do to fix it?"

Do you have the azalea in an appropriate environment (soil pH, sun exposure, water, etc.), does the soil drain properly, what kind of after care did the plant receive, what kind of azalea is it, and what is your location. That kind of stuff.

It's in a slightly raised be that gets sun most of the day. There are plenty of rhododendrons and azaleas thriving, close by and we are in the Lake District, UK with slightly acidic soil. We took a large phormium (8 foot high) that had been battered by heavy snowfall, out of that spot before planting the azalea. I can't remember the name of the type of azalea but it has creamy yellow flowers.

That sound like the fall foliage of many deciduous azaleas. All yellow azaleas are deciduous*, so you are probably just looking at a feature, not a problem. In the spring the new leaves are a beautiful green, slightly wrinkled and slightly hairy.

*There are a number of evergreen azaleas with flowers of light cream to greenish yellow, but nothing approaches the yellows found in the deciduous azaleas.

The azalea is Knap-hill-hybrid Daviesii and the foliage looked bad in July so I don't think it was normal leaf fall. Unfortunately I didn't see it earlier in the year as I was away.

The reddish brown foliage is the normal fall foliage color. If a plant is not happy in its situation such as not enough light or water, it will go into dormancy early and show this fall foliage.

I also have a sick azalea.
I'm in the Atlanta area. It's a well established, mature azalea. It's trimmed, but even being trimmed, it's 5' tall. It's a pink/red one time spring bloomer. It gets probably a little more sun than you'd prefer an azalea to get, but given its size and that it's established, I don't think that's been a problem. It's well mulched with cyprus, and previously with pine needles. Evergreen.

We moved into the house in April, so I don't know the variety or exactly how old it is.

I cut a small dead brown spot from it in May or so, but in the last few days I noticed this large spot- probably 20% or more of the plant. Is there anything I can do to save the plant?