In an attempt to cut costs (I use the Siligum quite a bit, mainly to reproduce small repetitive castings or details like these roof tiles or the window castings seen above), I purchased this alternative two part casting putty from e-bay for £19.50. The putty works in the same way - you mix equal parts of both putties and have a working time of about 5 minutes. The big difference between the two products is that while the Siligum is flexible, the Hard Rubber Putty is (as it says on the tin) hard! Very inflexible and even 'biscuity or crumbly'. There is no way that you can 'flex' this set material - it would just crack. The hard rubber putty is available from Restoration Supplies.
There may be the possibility of mixing both products, to produce a cheaper mould, but also one that does not crumble. However the costs of the raw material make this too expensive to risk loosing a whole mould.