Louise, I'm not quite following the second part of your question.
Customizing the splash page is a fairly simple process and there are examples available on CCO. Assuming you are using a WLC there is a webauth bundle that is located on the WLC download section, inside this bundle there are various options for page layouts as well as an instruction file letting you know how to use them. The main thing to remember is that the file size is limited of the tar file that you create and you need to use 7zip for example on Windows to create proper bundle.
ISE also has the ability to host the captive portal as well and there should be an example of that floating around on CCO as well I think. The same concept applies, you modify the HTML pages to your liking, tar them up, and then upload to your ISE box.
+5 Blake ... I didnt know NCS could host the guest page. So is that coin as the "external" server option under guest ?
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