Rather depends what unit you actually have Ewen. As per Musky's '??'. Some types resist reverse hook-up no problem, others may not, so it certainly helps to know.
Some, like DVR2s, are marked P or N, and an additional clue is in the colour coding of the wires that go to earth - there's a red for + earth, or a black wire for - earth.
V Reg 2 units have gone through colour scheme wiring changes - recent ones have the same red for + earth and black for - earth as DVR2s, and Green, Brown and Yellow for F, A and D respectively. Earlier ones are a bit more complicated as neg earth ones use Red for the A lead and Black for earth (with White for F and Yellow for D), Pos earth ones have Black for the A lead and Red for the earth, with Blue for the field and Yellow for D. Both types havibg a red was presumably confusing, so they changed it!
Podtronics ones say on them 6-PE or 6-NE for Pos and Neg I think.
The cheap and cheerful ones Wassell sell are wired differently at the dynamo for + and - earth, so if you've got one of them and can't see from the box which way is earth, you can look at the wires under the dynamo cover: for Neg earth the 2 field coil wires that emerge from the inside of the dynamo will be connected one to F and the other to D; for Pos earth, one will be to F and the other to Earth.
