Success! (I think)

Step one. I added two model parts, a command cupola and an external fuel tank, to a sprue.
 
Step two: I built the mold box using perspex.

 I wasn't sure how to proceed from this point because the instructions I was following called for the sprue to be placed in a bed of modelling clay, like plasticene or blu-tack and when I tried this, the pressures involved snapped the glued joints of the sprue. I decided to adopt a different approach and put a foot on the sprue so it could stand on a 1 cm bed of silicone.

Step three: Having laid a foundation of silicone, which I left to cure over night, the sprue was placed on top.

Step four. I needed keys, so I built a Lego frame and placed it along side the sprue.

Step five: I poured the second layer of silicone, making sure to be careful and avoid pouring the silicone over the model. This was quite difficult as the silicone is not an easy substance to control. I spilled quite a lot. I also found that my calculations were out by a good 25 grammes, and had to mix up a second batch in order for the silicone to reach the half way mark. Having completed this, I left it to cure over night again.

 Step six: After having applied the release agent (Dow Corning 7) to the second layer, being careful to make sure the keys were well coated, I poured the last layer of silicone and let it all sit over night. In the morning, I was pleased to find the mold came apart with a satisfactory squelching sound, and everything seems to be in order. You can see on the image how the accumulation of release agent in the keys has softened the edges of the lego bricks. It is noticable that inside the depressions made the model parts, there is no such softening.

My improvised mold making work station in the attic










My improvised gas mask - I've ordered a real filter mask, the chemicals are pretty nasty, but it hasn't arrived yet.