So using a technique to texture styrofoam taught to me by Andy (check out his excellent painting and terrain making blog here http://napoleonicwargamingadventures.blogspot.co.uk/ ) I came up with a new plan.
With mdf bases for all the ogres cut out, I marked the 25mm thick styrofoam with half of them as I intend to split the styrofoam once textured. It is a good idea to mark the styrofaom slightly larger than you want as the texturing will remove a bit of material.
The knife blade held rather too casually in my hand is a bread knife. I have bought this specially for such job as she who must be obeyed would go ballistic if I borrowed one from the kitchen :)
The technique is to drag the blade along the cut edge of the styrofoam to roughen it up. You can also use the point to dig out some larger chunks.
Second step is to take off all the flaky bits. I do this by rubbing my thumb over the surface till it is the texture needed.
Now I have all the pieces textured it is time to split the bases to get the number I need and a more sensible 12mm height
Voila! As our french comrades say! PVA glue to be applied and then leave them for 24 hours. Once the base is securely glued to the styrofoam I will 'paint' the textured edge with PVA to help stiffen this surface in case any loose bits were tempted to fall off.