Is it all corners or just some? certain carrying too much speed in can lead to under-steer ... As stated above there are a number of ways to correct under-steer simplistically it requires more grip on the front or less grip on the rear, so try any / all of the following:- get instruction from track specialist, Hawthorn will show up any handling issues, well set up Caterham is great from Tower through Jim Clark and Barcroft check rake, most aim for 15mm adjust / reduce tyre pressure, is the 20 quoted cold or hot ( does the under-steer build with the number laps?) adjust the rear anti roll bar (stiffener will create more over-steer to balance the under-steer) adjust the dampers trying doing one thing at once and feel the effects at one or two corners ... enjoy and keep it on the black stuff!