Thank you, Skell. Unfortunately, it looked so hopeful at first when I clicked on the link you gave but then it went to the blank 'add worldpay' page and I can do nothing else.
When I ctrl-u as bigmoe advised, it comes up with this (again, I deleted the numbers down the bottom and replaced with 'x' just in case):
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Add Worldpay</title>
<link href="/assets/sl-cashier.css?v=541" rel="stylesheet" type="text/css"/>
<link rel='stylesheet' href='https://payments.worldpay.com/resources/hpp/integrations/embedded/css/hpp-embedded-integration-library.css'/>
<script src='https://payments.worldpay.com/resources/hpp/integrations/embedded/js/hpp-embedded-integration-library.js'></script>
</head>
<body>
<div class="topbar" data-scrollspy="scrollspy" >
<div class="topbar-inner">
<div style="margin-left:15px; margin-right:15px;">
<img id="cashierLogo" src="/assets/sl-logo.png?v=541" alt="Second Life Cashier" />
<div id="user">
Welcome, <a href='https://secondlife.com/my/account'>AuraDey</a>
</div>
</div>
</div>
</div>
<div id="mainContent" class="container">
<div id='worldpay-iframe'></div>
<script type='text/javascript'>
var customOptions = {
type: 'iframe',
iframeIntegrationId: 'libraryObject',
iframeHelperURL: 'https://www.example.com/helper.html',
iframeBaseURL: 'https://www.example.com',
url: 'https:\/\/payments.worldpay.com\/app\/hpp\/integration\/wpg\/corporate?OrderKey=SECONDLIFEUSD%5Ed537c1b7-8883-4144-ac2a-b6f68519e150\x26Ticket=00149917101755102oET4UMI4DVjehwBvu8L4Tw\x26source=sc2',
target: 'worldpay-iframe',
debug: false,
language: 'en',
country: 'au',
preferredPaymentMethod: '',
successURL: 'https:\/\/accounts-customer.secondlife.com\/payment-method\/card\/xxxxxxxxxxxxxxxx/success',
cancelURL: 'https:\/\/accounts-customer.secondlife.com\/payment-method\/card\/xxxxxxxxxxxxxxxxxxxxxxxxx/cancel',
failureURL: 'https:\/\/accounts-customer.secondlife.com\/payment-method\/card\/xxxxxxxxxxxxxxxxxxxxxxxxxx/failure',
pendingURL: 'https:\/\/accounts-customer.secondlife.com\/payment-method\/card\/xxxxxxxxxxxxxxxxxxxxxxxxxx/pending',
errorURL: 'https:\/\/accounts-customer.secondlife.com\/payment-method\/card\/xxxxxxxxxxxxxxxxxxxxxxxxxxx/error'
};
var libraryObject = new WPCL.Library();
libraryObject.setup(customOptions);