Once I was happy with the shape I glued all the sheets together using PVA (you MUST use PVA, its NON-CORROSIVE to polystyrene unlike other glues), obviously leaving the sheet with markings on top XD
Once the PVA had dried ish(it never really did) I began cutting some of the excess polystyrene away with a saw. Using a saw will save you lots of time just dont cut close to the line - probably just cut 2/3 of the excess polystyrene away.
I then used a Rasp for the rest of the shaping, this took away loads of polystyrene in a short time, was easy to use and got a great result (thanks Dad for suggesting it!). After using the rasp I used the electric sander to finish off the look and smooth down the edges.
The pictures show the arm at different points in the shaping, it doesnt really matter too much if the original shape is rough as you can sand it into shape.
The claws were separately in the same way as above, I drew templates on paper then drew round them on the polystyrene.