Incorrect handling of 9 digit account numbers for non-Santander sort codes

[dancannon]

Hi, I am leaving an issue on a few of the modulus checking libraries on Github as they all seem to have the same issue (including our implementation at Monzo Bank). While the Vocalink modulus checking requirements specifically call out that Santander is the only bank that has 9 digit account numbers we have noticed that in practice other banks may also use 9 digit account numbers.

Specifically Coventry Building Society has this information in their FAQ

We use 9 digit account numbers - simply remove the first digit when giving your account number

The best way we thought of handling this would be to specifically use the documented logic in the modulus checking requirements only for sort codes used by Santander. For other 9 digit account numbers we simply remove the leading digit.

Based on this page we have defined this range as

090000-091900
720000-729999
890000-892999
165710

I hope this information helps, let me know if you have any other questions!