In reply to Bob Philpott:
If all your shots are over-exposed Clare, you probably have exposure compensation set to + something. Positive exposure compensation is useful for dark birds against light sky (eg Marsh Harriers) but for gulls, which are light anyway, leave exposure compensation set to the middle, possibly even under-expose slightly if the highlights are still blown. If you haven't already done so, go in to the settings on your 7D and enable Highlight Alert (it's the second blue menu, at the top). When you then view your images on the back screen, the blown highlights will flash.
The exposure compensation can be checked through the viewfinder (graduated scale along the bottom, runs from -3 to +3, you're after the mark to be in the middle). It will show up when you half press the shutter button. Alternatively, press the Q button and look at the rear screen. The same -3 to +3 scale will show up on there. Change it by scrolling the rear wheel whilst looking through the viewfinder (remember to half press the shutter button for a moment to start metering)
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