Finally got to the bottom of it - it is genuine. I posted a note to ntl using their web-based form and in the autoresponder reply was the following text:-
Latest News and Ongoing Issues
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22 Aug 2006
If you receive an email that has the basic header like this:
From:
do_not_reply@ntlworld.com
Date: 2006/08/22 Tue AM 11:06:43 GMT
To: Ntlworld email Customers
Subject: Join our email trial - your chance to shape our future
Please be advised that it is genuine, and is an invitation to trial a new web based email service.
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Now to decide if I can be bothered to do it after all that hassle!
Jim