Not sure it counts as an answer, but what we did to step round this issue was to move the bulk of the functionality - where the error handling was required - into a new business process, leaving only the most basic "pass the trigger message along" functionality in the business service. Added an extra component to the production, but we can now see errors in the log when they occur, and they are passed appropriately to Ens.Alert.

Follow-up, as a quick and dirty check, I replaced the call to GetValueAt above with:

set step1 = $PIECE(message.RawContent,"OBR|")
set step2 = $PIECE(part1, "ORC|", *)
set ReportId = $PIECE(part2, "|", 3, 3)

This works before calling the XML generation code when GetValueAt doesn't - so GetValueAt is definitely doing something to the contents of the HL7 message....

Thanks, helpful to explore another potential option. If I'm reading https://docs.intersystems.com/iris20233/csp/docbook/DocBook.UI.Page.cls?... correctly then we'd need to use the OnProductionStart method of the business process to run when the job is scheduled - and not sure that's obviously accessible in a BPL business process. There are otherwise no incoming messages to trigger action.

Can I clarify: "Run at a scheduled time" could mean either:

1. Run during a designated period of time, thus making the service / business process available to be triggered at anytime during that period but not outside that period, or
2. Run once at a specific time, and never otherwise be triggered.

We are interested in the second of these - we want a batch job to run at 0800, at 1300, and at 1600 each day. When it runs it will scoop all the current labs results out of the database table where they have been accumulating, and send them on to the downstream system.

Thanks for this. Yes, Schedule is there in Ensemble 2018.1 as an additional setting. We will investigate and see whether its a suitable/cleaner alternative to Ashok's suggestion.

Thanks @Ashok Kumar , with a bit of tweaking that got us to where we needed to go. For clarity for anyone coming along and reading this later the steps would be:

1. Create the BPL Business Process you want to call - at least have something in place you can call
2. Create the HL7Task.Test class in ObjectScript from the example above - renamed as appropriate. The argument to CreateBusinessService is the name in the Production of the Service you will be calling (created below). Also you need to set pInput as a class suitable for use as a request, such as Ens.Request or (we used for an example) Ens.StringContainer
3. Create the HL7.Feed.TriggerService class in ObjectScript from the example above, renamed as appropriate. 
4. In the Production, create a new service (whose name is that used in step 2), and whose class is the one created in step 3. Set the TargetConfigName on the service to the name of the BPL. Set the Pool Size to zero.
5. Create a new scheduled task - run it in the appropriate namespace, and pick the class name from step 2 in the Task Type dropdown - and schedule as appropriate.

Once all this is in place, you can call a BPL Business Process from a scheduled task.

Thanks Ben, helpful answer. Another reason to try and kick our organisation towards upgrading to Iris 😉

Keren, great, thanks - this has allowed me to see the content of the messages and I can dig further.

Thanks, yes, this is what it needed to be - got there myself, as it happens, but thanks. Documentation wasn't very clear. 

Carmen, thanks, that's really helpful. If we need further information we'll reach out - as is so often the case, I'm doing some technical investigation for a development that may not ever be tackled, or may be tackled in a different way, or whose specifications will change wildly before we tackle it, or...!!!

Alex, thanks. Yes, license is still valid. And no, not possible to suspend before changing problem item - getting databases locking errors when I try. No idea how we managed to corrupt this quite so thoroughly!!

Thanks Alexander. Terminal Task Manager:

• can delete other tasks, but
• trying to delete the "corrupted" one get: "ERROR #5803: Failed to acquire exclusive lock on instance of '%SYS.Task'" (same as when using Management Portal)

Re figuring our where system tasks are stored via journalling, I understand the principle of what you are saying but we are probably reckoning the effort in doing that at least as great as scrubbing and reinstalling - we lose some config (we've got it documented, but the developer who did it originally has left), but no important running code.