This issue has been discussed in a couple of other reports (see http://forums.filemaker.com/posts/543eabe140). Setting the field to be index in unicode seems to be a viable workaround until this bug gets fixed.
For More Information see: Value list not working when using "related values only" from date field
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks to you both for this. I am sorry I did not find it in the previous reports. Reassuring to see that it is a known issue. Hopefully this means that a fix is on the way. I may hold off buying FM 12 until then.
Playing with the demo has also given me a chance to look at the way FM 12 works with the iPad version. If they can sort out a better way of keeping the Mac and iPad versions in sync then I am definitely sold. I understand that this may be a Mac OS or iTunes issue and out of the hands of Filemaker, although many other companies have started using iCloud. I suspect I'll be waiting quite a while for this.
At least 2 3rd party companies: 360works and SeedCode offer synch utilities for Go.