Have a look at the Hitech 100x150mm graduated filter range (they have both hard and soft). Would recommend a set of 0.3, 0.6 and 0.9 (hard or soft) filters. They also have a low profile holder for wide angles, but this has less stacking possibilities than the normal one. They are a bit less expensive than Lee, but more expensive than Cokin (which may not have a complete neutral effect, causing a bit of colour cast in some situations).