Rather than serializing the result manually just return the type directly from GetDepositAccountStatementViews like a normal method.  This is a very common mistake for some reason...

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public TheType GetDepositAccountStatement(long accountNumberId, string fromDate, string toDate)
{
	var frmDate = DatabaseLayer.Core.DateConversion.GetEnglishDate(fromDate).ToDateTime();
	var tDate = DatabaseLayer.Core.DateConversion.GetEnglishDate(toDate).ToDateTime();
	return BusinessLayer.Deposit.DepositAccountStatement.GetDepositAccountStatementViews(accountNumberId, frmDate, toDate);          
}

Other than that, the columns returned do not match up.  There is a case issues and one column, statement_reference, does not match.   Take a moment to review your code.