Right! Closure - have heard back from the Museum of Design in Plastics. They are reasonably certain that the cocktail shaker is made of urea formaldehyde.
'It is very similar to this Cream and green coloured dressing table pot | Museum of Design in Plastics which we hold in our collection. If you are interested to know a little more, this link describes the essentials of this material: http://www.modip.ac.uk/resources/curators_guide/A-Z_plastics#U-Z.
There you go. Case closed! Thanks all.
