Being rather impatient I reverted to a 32bit version of Ubuntu and installed the 32bit version of NoMachine (i386). It works! So my conclusion is that while you may be able to run 64bit Ubuntu on a 32bit machine, you won’t be able to run 64bit NoMachine.
This didn’t take very long so if you want me to do some tests I don’t mind.
Regards, Geof