I spotted a note on ITV Teletext itself: an offer of a free guide. I sent a request email to them asking for one but the message failed to send, leading me to think that the service has been discontinued. I will look to follow this up, maybe writing to the address though I am not hopeful of getting a reply.
On a more positive note, I did find a guide from 1995:
Source, and a higher res version - mb21