How do you catch big carp amongst smaller carp?

There is a lake that I often fish (UK) that has plenty of 20 lb'ers in it, but I often get pestered by smaller fisharound the 8 lb mark. Is there any way of attracting more double figured fish and 20 lb'ers into the swim? If I chuck out loads of groundbait, will that attract bigger carp as well as smaller carp?