Gorilla Super Glue. http://www.gorillaglue.com/glues/superglue/index.aspx (http://www.gorillaglue.com/glues/superglue/index.aspx)
It expands slightly and sticks hard plastic very well.
They do other types of glue too, but I haven't tried them yet.
Where can you buy it?
Most DIY places have it or you can buy it online from sites like.
http://www.gluegorilla.co.uk
http://www.gorillaglueonline.co.uk