Hi Matt,
good to hear from you and I hope all is well.  This question is about SDB of course! 
I hadn't thought of REST as a route to the iteration, so I'll have a poke around the docs - I think https://jazz.net/wiki/bin/view/Main/ReportsRESTAPI#Reportable_REST_API is propably a good starting place.  I'd prefer not to mix and match methods, but if it gets me to where I need to be then so be it.

Yes, looks like REST is the way to go.  Here's how to get the full picture of a plan (mine is called "Next SDB".  The double quotes got translated into %22) :

https://<ccmserver>:9443/ccm/rpt/repository/apt?fields=apt/iterationPlanRecord[name=%22Next%20SDB%22]//

And to get just the iteration ID:

https://<ccmserver>:9443/ccm/rpt/repository/apt?fields=apt/iterationPlanRecord[name=%22Next%20SDB%22]/iteration/id

Thanks Matt - I'll call you soon - and if there’s a way to get this via the normal Java API I'd still like to know.

With some more digging I eventually found a way to do this using Sam's code in this thread as a guide: https://jazz.net/forum/questions/94776/assertionfailedexception-problem-with-getting-the-values-of-attributes

This is my version, which gives me exactly what I want:

    public static IIterationPlanRecord getIterationPlanRecord (ITeamRepository repo, String planName,
            BufferedWriter bw, Boolean debug)
            throws TeamRepositoryException, IOException {
       
        if(debug){bw.write("getIterationPlanRecord Entry\nLooking for Plan \'"+planName+"\'\n");}
       
        IIterationPlanClient planClient = (IIterationPlanClient) repo.getClientLibrary(IIterationPlanClient.class);
        IAuditableClient auditableClient = (IAuditableClient) repo.getClientLibrary(IAuditableClient.class);       
       
        List<IIterationPlanRecordHandle> phandles = ((IterationPlanClient) planClient)
                .fetchAllIterationPlans(ItemProfile.ITERATION_DEFAULT, null);
        List<IAuditable> itall = auditableClient.fetchCurrentAuditables(
                phandles,
                ItemProfile.createFullProfile(IIterationPlanRecord.ITEM_TYPE),
                null);
     
        int i; for (i = itall.size() - 1; i >= 0; i--) {    // Loop from last to first
            IIterationPlanRecord ipr = (IIterationPlanRecord) itall.get(i);          
            String thisPlan = ipr.getName();
            bw.write("Plan Name: "+thisPlan+"\n");          
            if (thisPlan.equalsIgnoreCase(planName)) {
                if(debug){bw.write("getIterationPlanRecord Exit\n");}
                return (ipr);
            }
        }          
        if(debug){bw.write("getIterationPlanRecord Exit\n");}
        return (null);   
    }