particularly the last link on the page -- http://www.jgroups.org/javagroupsnew/docs/manual/html/ch02.html#ItDoesntWork .
Also useful is http://www.29west.com/docs/THPM/multicast-address-assignment.html
The result of the 2nd URI works one way and not the other. Hmmm. Will investigate further.
Again, thanks - I'll keep digging (my SysAdmin's gone home :-)
Working ... once sysadmin managed to climb out of bed and remember he'd disabled something in the ip tables!
Thanks for the pointer tho' - the test programs are a boon, and i will (no doubt) be using them again.