Sorry but I think that you're going to be out of luck here. Reasons:
1. Nationwide was normally transmitted live. Its early broadcast were probably before the days of videotape and its latter days would have been when videotape was really expensive so that everything was wiped and re-used. (The BBC occasionally filmed parts of programmes for its archives but I doubt if a complete programme has survived).
2. The BBC gets so many requests for these sort of things that it generally works to a policy of simply saying 'Sorry, we can't help'.
3. Even if the BBC could track down a copy of a particular programme, there would probably be copyright issues in making a copy for use outside of the BBC.
So, it's unlikely that you'll be able to track the programme down. All may not be lost, however. It could be worth contacting the National Film & Television Archive. Use the form here: (Check that the pull-down box shows 'NFTVA Enquiries') -
http://www.bfi.org.uk/help/contact.php?eid=53
Chris