I discovered that if you have to bring your car back for a visual test you have circa 30 days to do it.
If you have also made an appointment to have the under carriage of your car tested as part of the 2nd test debacle but that this test is outside the 30 days then the "visual" test when completed.....will cancel your undercarriage test. You have to re apply again for the under carriage test , pushing this 2nd test even further out, time wise.
It seems that their system will only allow the tester to pass/cancel one test at a time.
Happened to me.