So what have I learnt?
Positives:
Reusable
Cheap
Usable with both Green/Grey stuff and Milliput
Negatives:
Hard to line up both sides of the mold 100%
Milliput sticks to the mold
Needs boiling water
Milliput takes along time to set
Soft details when used with Milliput
Well anyway I've made 4 molds and 9 casts made from Milliput (I now have made another 3 molds but are untested), 3 casts came out well enough to use. 6 were misaligned at best.
I'll take some pictures when they are painted next to the originals, but details are alittle soft so I'm going to switch to Greenstuff after the rest of the Milliput is used up and hopefully that'll improve the cast rate.