class DirectDebitForm(forms.ModelForm):
    terms_and_conditions = forms.BooleanField(label="Tick here to confirm you accept the Terms and Conditions")
    # overriding model field as need number validation before clean() invoked
    account_number = forms.CharField(min_length=8, max_length=8, label='Account Number   (8 digits)',
                                     validators=[RegexValidator(r'^[0-9]{8}')],  # making only numbers valid
                                     error_messages={
                                         'invalid': 'Your account number should be 8 digits with no spaces or letters.'})
    sort_code = forms.CharField(min_length=6, max_length=6, label='Sort Code   (6 digits)',
                                validators=[RegexValidator(r'^[0-9]{6}')],  # making only numbers valid
                                error_messages={
                                    'invalid': 'Your sort code should be 6 digits without dashes or spaces'})

    def __init__(self, session, my_quote, *args, **kwargs):
        super().__init__(*args, **kwargs)
        ......

    def clean(self):
        cleaned_data = super().clean()
        account_number = cleaned_data.get('account_number')
        sort_code = cleaned_data.get('sort_code')
        if account_number and sort_code:
            # validation via external api
            error, bank_name = get_bank_validation(account_number, sort_code)
            cleaned_data['bank'] = bank_name
            if error:
                if error == 'no-branch-found':
                    self.add_error('sort_code', 'No bank could be found for that sort code. Please check.')
                elif error == 'account-number-invalid':
                    self.add_error('account_number',
                                   'Invalid account number. Please check your account number and sort code.')
                else:
                    raise forms.ValidationError("The account number and sort code are not valid. Please check.")

    class Meta:
        model = DirectDebit
        fields = ['account_name', 'account_number', 'sort_code', 'bank']