Next would be to Chip it / Re Map. Then onto some cams from the likes of Courtenay Sport. Here is a rough guide to power and price for tuning it up a bit more. If I was you I would buy a decent exhaust first then get a re map..
CS1
Remap
Increases to +8 bhp £295 Fitted
CS2
Remap, Airbox Mods, Sports Air Filter
Increases to +10 bhp £350 Fitted
CS3
Remap, Airbox Mods, Sports Air Filter, Milltek Stainless Steel Sports Exhaust (Tailbox and Centre Section including Sports Cat and pre-cat removed), Rolling Road Tune
Increases to +20 bhp £1,558 Fitted
CS4
Airbox Mods, Sports Air Filter, Milltek Stainless Steel Sports Exhaust (Tailbox and Centre Section including Sports Cat and pre-cat removed), Modified Cylinder Head and Remap, Rolling Road Tune
Increases to +28 bhp £2,610 Fitted
CS5
Airbox Mods, Sports Air Filter, Milltek Stainless Steel Sports Exhaust (Tailbox and Centre Section including Sports Cat and pre-cat removed), Uprated Schrick Camshafts with double valve Springs and collets, Modified Cylinder Head, Remap, Rolling Road Tune Increases to +35 bhp £3,600 Fitted