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I am looking at getting a GPS nav unit sometime soon, but find the choices largely varied, do you have any advise for what sort of things to look for and things to avoid?

Of course a free Navman would be cool, but just in case i don't win advise for buying would be appreciated.

Look for AA certified maps. Consider how you want to use the nav unit.
There are many different models from each brand and the differences are a lot more than just screen size. Different brands and models have different features.
Navman and TomTom have very good websites which explain what their different models do. There is no best brand or model and new ones come out frequently. The question is what do you want to use it for. For example is real time traffic important? Do you travel for business or pleasure? Do you want something portable that you can take overseas or use in other vehicles? Do you need a large display?