I decided to ring the Application Helpline to find out why and rectify the situation. They were "unable to discuss the in-house criteria with the customer" but did suggest that common issues are "not living for a long time at your current address" and "not being with either your current bank or job for a significant period".
"OK" I said. "Can I supply you with details which might help rectify that and verify my background?".
"No" said the voice at the other end. "What you put on your form is confidential, and we can't discuss it".
So I'm left wondering what this so called 'helpline' can help with. I'm also wondering why I bothered to verify my identity if they clearly weren't going to discuss anything with me anyway. Honestly, you stand just as much a chance as I did of getting something useful from them.
What a waste of time and space.