what is the best way to do the studs that appear on the mk 5 and mk 6 armour?

(preferably methods other than using plasticard)

Try straight pins, as used in sewing. Use a small bit in a pin vice to drill a small hole into the model. Then, cut the head of the straight pin off of the shaft, leaving just a bit of the shaft attached. This can then be glued into the hole you've drilled into the model.

Using this method may be a bit more time consuming (barely) than using plasticard discs, but results in more 'realistic' and uniform studs. It works great on Marine Power Armor, but really sucks when trying to make service studs on the heads of helmet-less Marines.

go online and get the golden Eagle leather punch as shown in the gw catalog.
I picked one up the make perfect studs for men and vehicles.

http://goldeneagletradingpost.com/toolshides.html