Update:
Ran the BT WS Performance Tester on IE11, results obtained:
DS/US 35.52/1.67Mbps. (believed to be correct for my line, PN 40/2 package)
No warning given ref. Adobe Flash usage before test run.
Re-run tester using Firefox, latest revision, results obtained:
DS/US 17.77/1.57Mbps. (TBB desktop speed indicator showed approx. 35Mbps. DS during the test as per normal expectation)
Also noted recently that BT WS Tester "further diagnostics" process frequently failed to complete with Firefox, appears to proceed normally using IE..
Would appear that my issue is with Firefox browser and is associated with the appearance of the "Adobe Flash" message.