does anyone know where I can get a sheet of fibreglass or plastic 1mm thick in black?

In the Midlands, prefably from Wolverhampton??

Try roofing suppliers, it's used for flat roofs.

Mould the sheets yourself on a piece of alloy sheet?
(Using black gel coat and polyester resin and glassfibre matting).

It can be moulded on glass, or melamine

Try a local plastics supplier.

Laying up on a sheet of melamine will give you a slightly matt effect finish if thats what you are after but its dead easy to lay up on and more importantly dead easy to remove without the need for any waxing etc.