Thanks Troy.
I tested on a different laptop with 10.14 and the OnGuard agent correctly detects Filezilla, etc. so it appears to be a problem with the earlier 10.13 OS X system. I didn't appear to have any trouble modifying the policy but double-checked this through logging and in again, rebooting, etc. but this made no difference.
I'll log a TAC case as you suggested.
Regards,
John