OK after several failed attempts at making a silicon compound to make molds from i succumbed to the lure of ebay and purchased 100g of siligum paste for around £10. I was dubious but a little desperate as I had some bits I wanted to replicate and at that price decided it was worth the risk.
heres a pdf of my 'experiment' :http://www.mediafire.com/download/prx24vp7pn1bd71/OSJC_siligum_casting.pdf
oh and p38 car body filler makes a great cheapo resin for those of us on a tight budget!