[kiwi 911e]

Hi I need some help please.2007 COOPER S low speed surge problem.under low speed conditions it surges bad and runs rough.have changed plugs and coils but no improvement at all .have sprayed sea spray into inlet and added to fuel to clean carbon and still no improvement.car has traveled 90 000 km.the throttle seems to not respond when depressed slightly as well.It has no fault codes at all.I am thinking hp fuel pump but would like other opinions first and how do i test the hp fuel pump.I am a experienced mechanic but have not worked on these mini before.thanks.