Before you all say what a "cheap skate" I am - I have purchased a silicon mold and sent for some acrylic resin.
BUT was reading on the internet about using 50/50% mix of plastercine and milliput to make things in molds. So decided to try 50/50% mix of blue tack and household filler - first one was a blob of blue tack and tried to mix with the filler. Second one chopped up the blue tack and then mixed this with the filler - will let it dry for an hour or so and see what it looks like. At the moment don't think it will do well in the mold but if it works could make some larger blocks for fancy walls that I need in the next model instead of drilling, filling out of sheet acrylic.