Malcom - Don't waste money on a filter for your particular lens as H says.
If you're using Photoshop to process your images, there is a couple of CA removal/reduction tools in Noel Carboni's PS actions...
http://www.prodigitalsoftware.com/As...l_Version.html
Way cheaper ($21.95US) than a filter and you'll have fun with the other actions too!
Found specific one...
http://www.prodigitalsoftware.com/im..._Reduction.jpg
Cheers
Doug