Orb Web Spider?

Found this fantastic spider in grassland on the Downs this morning. It was quite large - I'd guess that the abdomen was a bit bigger than a 5p piece. The abdomen is a light yellow-green colour with four tiny dark spots on top. The legs have brown stripes. I assume, from its shape, that its a member of the Orb Web family but would really like to know which one it is. Any suggestions?