I have a few questions which I would be grateful if anyone can
answer:-

1) Should I lay the form board / shuttering down before I lay the
hardcore?
I wouldn't, I would compact the hardcore then put the formers on top. Set
the top of the formers to finished concrete floor level with a gap
underneath so the concrete flows under the formers to seal the top of the
hardcore.

(This means boards need to be about 8 inches high). Or
should I lay the hardcore first, then 4 inch high boards (nailed to
stakes driven into the ground). I am worried that if I put the boards
down first, they may split when compacting the hard core with a
whacker plate.
2) Is a 19mm thick board ok to use (for the shuttering)

So long as you use enough support stakes to keep them straight 19mm is
ample.

3) What do I do with the damp proof membrane after the concrete has
set - should I just cut the edge of it flush with the concrete?

I would bring it up the sides of the concrete if you can to stop damp from
the soil working in at the sides.

I have almost finished building a shed/workshop of my own
http://www.twango.com/channel/Muddymike.Workshop

Mike