[StuBeeDoo]

Thanks for your input, Joel.     Smooth-on is my go-to for tyre production products.  Not the cheapest, but I'm happy with it.

The Fastcast that John linked almost had me, but 2kg is far too much for me and they don't appear to have smaller quantities.

I could do with ordering more silicon from Bentley's so I'll give the Smooth-Cast 322 a go.