Right, after finally getting an email from technical support, the problem has been solved for me.
If you haven't been supplied with the fix, or looking here for help, here is how to sort the problem.
1. Go to the update card details where you input your new bank card info.
2. Input new card details.
3. Go to the billing address details underneath your card details.
4. Select the option to manually input your billing address.
5. On door number, this needs to be 3 digits long. So for examples:
If your door number is 8, enter 008
If your door number is 23 enter 023
If your door number is 315 enter 315.
Basically insert 0's before your number to make it 3 digits.
6.Enter the rest of your address in the other boxes.
7. Hit the submit button at the bottom of the page.
That should be your details now saved.
Thanks to the technical support for finally getting back to m with this fix.
Not an obvious solution, but worked for me.
Hope this helps anyone else having this issue.
All the best.
