I wonder if you can help?

I have created this webpage: www.ucsite.co.uk/index.htm

All my sections and divs are sized using percentages with the exception of the header which has a height of height: 17.5em. As soon as I change it to a percentage the whole site falls apart. This suggests to me I have done something wrong. It would be great if someone could point out what. I like learning from my mistakes

Many thanks,