Hi adrianp,
I don't have much useful to say except: make sure you are using up-to-date drivers for MySQL.
I recently ran into a similar issue and even ended up reporting a bug to the Groovy SQL folks, only to find out in the end that it was the old version of the MySQL JDBC drivers were buggy.
(See
http://www.nabble.com/Strange-issue-with-Groovy-SQL-parameters-tp18587220p18587220.html for reference).
Cheers,
Simon.