overexposure of sky, what to do

Have you tried using AEL (Auto Exposure Lock) to lock your exposure on a lighter part of the scene before taking your shot. It sounds to me like you are shooting into a backlit scene or into shadows that are causing you to over expose. If you read in your manual you can see how this feature works.

OK I have taken the same scene but first pointed at the sky and pressed ae-l then pointed at the scene as I previously over exposed and voila you are correct, problem solved. The sky is OK and the trees darker than before.

What I now need to know is what you mean by backlit. I assume you mean the light (sun) is virtually behind me lighting the scene in front of me. But I thought that was a normal situation.

The scene I am taking is a hill in front, covered in trees with a small area of sky above but the sun, if any, is behind me and is or isn't lighting the trees in front of me.

This would suggest that I always need ae-l.I would be grateful for a little guidance here.
Thanks
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For the bird's and small animal's sake, please cut all plastic loops in 4 can retainers before going to land fill.

Backlit usually refers to when the sun is behind your subject. It can cause real issues with metering as well as you may have a lot highlights and shadows which cause exposure problems.

It seems then that my scene is front lit. Any suggestions on why this happens then?
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For the bird's and small animal's sake, please cut all plastic loops in 4 can retainers before going to land fill.

When you look at a scene with the naked eye you need to understand how it is lit. A modern camera sensor can only capture about 4 - 5 stops of exposure while the human eye can process about 10 stops of exposure. Therefore we are always trying to find scenes that have decent balance. Good shadows and highlights, with a decent range of neutral lighting. Looking at a histogram you would see this as a scene with sharp curves peaking at about 80% on the left (dark side) and right (light side) of the histogram with the middle being at about 18%. The problem is that this is almost always not the case so we have to make some compromises. When there is an excessive shift to the left or right side of the histogram we will almost always have exposure problems. The key is understanding where the most balanced lighting point in your scene is in these circumstances and using something like AEL to lock your exposure on this spot and then taking your shot. You will still end up with lost details in shadows or overblown highlights even when doing this, but you should be able to fix these in post processing.

You may also want to experiment with bracketing at 1/3 stops. This will shoot an exposure 1/3 of a stop down, 0 and 1/3 of stop above your metered exposure reading providing you three photos to evaluate during post processing to try and drive out your best results. I find 1/3 of stop down (especially in bright scenes) gives me the best opportunity to get the best detail and lighting out of landscapes.

Here is a good introduction on how to read a histogram; you should really invest some time understanding how this useful tool works:

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

Cheers

Thank you very much for your help, now it is over to me and the article.
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For the bird's and small animal's sake, please cut all plastic loops in 4 can retainers before going to land fill.