After failing with the first experiment with the Lego blocks, I decided to have another go and see if I have learnt from my mistakes.
- watch out for leaks 
- apply Vaseline to all areas - liberally!
This time I used an ice-cream box and the empty plaster tub. Using the wet clay made a real difference too, much easier to work with.
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| after the plaster and clay were released from the tub, the hand fits perfectly back into the hole | 
Whilst I was waiting for the plaster to go off, I had a play with the left over clay, and made this:
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| Magical lady cat | 
 She is full of wisdom, very lucky and fertile!