You forgot to quote the name attribute as well. And you forgot that names and addresses might contain quotes.
if ($error != 1)
{
?>
<form name="form1" method="post" action="https://sslrelay.com/hartsnursery.co.uk/Reciept.php">
<input name="Submit" type="submit" class="Checkout" value="Complete">
( Click To Proceed )<strong>
<input name="FirstN" type="hidden" value="<?php echo htmlspecialchars($firstname)?>">
<input name="LastN" type="hidden" value="<?php echo htmlspecialchars($Lastname)?>">
<input name="Address" type='"hidden"' value="<?php echo htmlspecialchars($Address)?>">
<input name="Town" type="hidden" value="<?php echo htmlspecialchars($Town)?>">
<input name="Post" type="hidden" value="<?php echo htmlspecialchars($Post)?>">
<input name="Telephone" type="hidden" value="<?php echo htmlspecialchars($Telephone)?>">
<input name="CardH" type="hidden" value="<?php echo htmlspecialchars($CardH)?>">
<input name="CardN" type="hidden" value="<?php echo htmlspecialchars($CardN)?>">
<input name="Type" type="hidden" value="<?php echo htmlspecialchars($Type)?>">
<input name="Issue" type="hidden" value="<?php echo htmlspecialchars($Issue)?>">
<input name="Start" type="hidden" value="<?php echo htmlspecialchars($Start)?>">
<input name="Expiry" type="hidden" value="<?php echo htmlspecialchars($Expiry)?>">
<input name="Security" type="hidden" value="<?php echo htmlspecialchars($Security)?>">
<input name="Items" type="hidden" value="<?php echo htmlspecialchars($items)?>">
<input name="TotalCost" type="hidden" id="Items" value="<?php echo htmlspecialchars($_POST['TotalCost'])?>">
<input name="Ordern" type="hidden" id="Items" value="<?php echo htmlspecialchars($saleid)?>">
<input name="Email" type="hidden" id="Items" value="<?php echo htmlspecialchars($Email)?>">
</strong>
</form>
<?php
}
else
Some of the htmlspecialchars() calls can be removed if you know that their values cannot contain anything that might break the HTML (in this case, particularly, " and >).
A couple of other things I noticed: only one element can have an ID of a particular name; you have three. And is it my imagination or are you allowing people to set their TotalCost to anything they feel like?