Thanks Mark,
I had previously looked in BrokerLink control and it already had the ' update Level 1 at MTA' flagged 'No'.
I'm wondering if it could be linked to APM? - that said, I can't see anything in the 124 options within Prospect Control that may be the culprit. Plus it's hard to replicate in a test environment without mucking up live EDI cases.
It's not a mega urgent issue, so I'll see if anyone else on the forum can point me in the right direction before logging a support request.
Have a good day