Registration code not working

Either I'm just being completely blind or my code is refusing to work.

Can't seem to get my registration form to work.
I know that all the names are correct as I've submitted an empty form and I'm getting all the error
messages I wrote back. e.g username is missing

I've checked that all the database values are correct and are going into the right table and yet I'm just getting back "query failed" so I know for a fact that something is wrong with one of the two sections where I've wrote "query failed" but I just can't see the problem...Anyway, here's my code:

reg.php

<form id="loginForm" name="loginForm" method="post" action="regcheck.php">
  <table width="300" border="0" align="center" cellpadding="2" cellspacing="0">
    <tr>
      <th>First Name </th>
      <td><input name="FirstName" type="text" class="textfield" id="FirstName" /></td>
    </tr>
    <tr>
      <th>Last Name </th>
      <td><input name="Surname" type="text" class="textfield" id="Surname" /></td>
    </tr>
    <tr>
      <th width="124">Username</th>
      <td width="168"><input name="UserName" type="text" class="textfield" id="UserName" /></td>
    </tr>
    <tr>
      <th>Password</th>
      <td><input name="Password" type="password" class="textfield" id="Password" /></td>
    </tr>
    <tr>
      <th>Confirm Password </th>
      <td><input name="cpassword" type="password" class="textfield" id="cpassword" /></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td><input type="submit" class='button' name="Submit" value="Register" /></td>
    </tr>
  </table>
</form>
<?php
	if( isset($_SESSION['ERRMSG_ARR']) && is_array($_SESSION['ERRMSG_ARR']) && count($_SESSION['ERRMSG_ARR']) >0 ) {
		echo '<ul class="err">';
		foreach($_SESSION['ERRMSG_ARR'] as $msg) {
			echo '<li>',$msg,'</li>'; 
		}
		echo '</ul>';
		unset($_SESSION['ERRMSG_ARR']);
	}
?>

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and...

regcheck.php

<?php
	//Start session
	session_start();
	
	//Include database connection details
	require_once('config.php');
	
	//Array to store validation errors
	$errmsg_arr = array();
	
	//Validation error flag
	$errflag = false;
	
	//Connect to mysql server
	$link = mysql_connect(DB_HOST, DB_USER, DB_PASSWORD);
	if(!$link) {
		die('Failed to connect to server: ' . mysql_error());
	}
	
	//Select database
	$db = mysql_select_db(DB_DATABASE);
	if(!$db) {
		die("Unable to select database");
	}
	
	//Function to sanitize values received from the form. Prevents SQL injection
	function clean($str) {
		$str = @trim($str);
		if(get_magic_quotes_gpc()) {
			$str = stripslashes($str);
		}
		return mysql_real_escape_string($str);
	}
	
	//Sanitize the POST values
	$FirstName = clean($_POST['FirstName']);
	$Surname = clean($_POST['Surname']);
	$UserName = clean($_POST['UserName']);
	$Password = clean($_POST['Password']);
	$cpassword = clean($_POST['cpassword']);
	
	//Input Validations
	if($FirstName == '') {
		$errmsg_arr[] = 'First name missing';
		$errflag = true;
	}
	if($Surname == '') {
		$errmsg_arr[] = 'Last name missing';
		$errflag = true;
	}
	if($UserName == '') {
		$errmsg_arr[] = 'Username missing';
		$errflag = true;
	}
	if($Password == '') {
		$errmsg_arr[] = 'Password missing';
		$errflag = true;
	}
	if($cpassword == '') {
		$errmsg_arr[] = 'Confirm password missing';
		$errflag = true;
	}
	if( strcmp($Password, $cpassword) != 0 ) {
		$errmsg_arr[] = 'Passwords do not match';
		$errflag = true;
	}
	
	//Check for duplicate UserName
	if($UserName != '') {
		$qry = "SELECT * FROM user WHERE UserName='$UserName'";
		$result = mysql_query($qry);
		if($result) {
			if(mysql_num_rows($result) > 0) {
				$errmsg_arr[] = 'Username already in use';
				$errflag = true;
			}
			@mysql_free_result($result);
		}
		else {
			die("Query failed");
		}
	}
	
	//If there are input validations, redirect back to the registration form
	if($errflag) {
		$_SESSION['ERRMSG_ARR'] = $errmsg_arr;
		session_write_close();
		header("location: reg.php");
		exit();
	}

	//Create INSERT query
	$qry = "INSERT INTO user(FirstName, Surname, UserName, Password) VALUES('$FirstName','$Surname','$UserName','".md5($_POST['Password'])."')";
	$result = @mysql_query($qry);
	
	//Check whether the query was successful or not
	if($result) {
		header("location: register-success.php");
		exit();
	}else {
		die("Query failed");
	}
?>

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If you can see the error, please do let me know as it's driving me mad.
As you can see, the  "Check for duplicate UserName" and "Create INSERT query" are the two
sections with "Query failed".

Thanks.