That's a Dragon Tree Palm isn't it? I don't see anything wrong with it. The lower leaves naturally turn brown and fall away as it grows. The newer growth looks firm and green. That's what's happening, your plant is growing. I see no signs of pest damage. Be careful not to over-water and avoid direct sun.

Here are some tips:

http://www.guide-to-houseplants.com/dragon-tree.html

The yellow spots on the green leaves may be an indication of an insect pest, such as mealybugs or scale, at work deeper in the whorl of new leaves. The mold on the older leaves would then be growing on the sugar syrup they secrete. If the bugs are accessible enough, they can be killed by touching them with a cotton swab soaked in meths. More hidden ones need a spray of mild organic soap directed into the center whorl. Repeat as necessary--scale can be particularly persistent!

I can't see any obvious bugs but will check again thanks. The plant was always fully green and only lost a rare leaf, I've had it several years. Now it is clearly dying, something is attacking it, either bugs or fungus. It's an outdoor plant, not a houseplant.

It's a cordyline. Cordylines are trees (palms), so the loss of its lower leaves as it grows is natural, thus revealing the trunk. If you do a google search, you'll see what I mean. They become very tall, so you'll eventually have to plant it in the ground (if you want to keep it!).

The yellowing on some of the upper leaves is due to lack of nutrients. Potted cordylines make very dense root systems, so they need lots of feeding and watering.

I don't know whereabouts in the UK you are, but I'm in south-east Wales, and my cordylines (in containers) stay out all year round without protection of any kind.

Wrong time of year to feed it, but, although its hard to tell because the pics are sideways on, it looks to me like its needed a much larger pot for some time... that would explain its condition, and why its looking nutrient poor... it doesn't have enough root room. You've also said you've had it for 'several years' and after several years in the ground, it would have a trunk that's at least 5 or 6 feet tall, so its growth has been very curtailed. I don't suppose you've got room in the garden to plant it out in a sheltered spot? And yes, I confirm its a Cordyline australis...

When you get it a bigger pot don't get one the same shape. i was seduced by one that shape without thinking - there's a lot less root room than in a more straight sided one, so you need to pot on a lot sooner. You'll probably find that the roots are all scrunched up and longing for more space.