What are you using it on? Selleys do a product called Plastibond that may suit your needs and it's available from your local hardware store or if your looking for a large quantity of filler then talc is often mixed with polyester (fibreglass) resin.
Thanks gizmo. Im just repairing some small holes to the bottom of my fibreglass boat and a friend of a friend gave me a little of the fibral to use and it worked a treat. So I thought I would buy some as I have found some more sections to repair. Im hopeless with mixing stuff and this seemed quite easy to use.