OK, it's working for me now.
Overall, it's an excellent service. I have made the following suggestions
1) Text and email reminders are fine, but if someone has not booked a test within 30 days of their NCT expiring, they should be sent a reminder by post
2) As there is a huge seasonal issue due to cars being sold in January, change the requirement for cars between 10 and 15 years old to 18 months .
3) I hadn't realised that there was an Appeals Process. It needs to be advertised more. Maybe put a note on failure notices if it's not there.
4) The carry on about seat belts not being visible should be fixed. The driver should be allowed to fix the problem and be retested instantly.