Hp1982 unfortunately that will depend on which unit !
The reason is historically all Police HEMS were ex millitary and as such the requirements were set by them.If you consider the average requirements for this type of flying then it is still quite difficult to obtain them if you're a civvy trained pilot.Specifically the twin time and the night time.It is however possible if you have a north sea background,but I would suggest that a purely north sea background (flying a helicopter like a plane) isn't wholly appropriate either.Hence a requirement for some form of on shore experience.What actually counts may well as I suggested depend on the interpretation of the unit.Best bet ask the chief pilot of the unit you're looking at.