Brokers should have a MCA "Bill of sale" if the boat has been through a broker: http://www.dft.gov.uk/mca/uksr-msf4705.pdf
Some private sales do some don't.
Although the below are for Salty boats, they can also apply to narrow boats
http://www.ybw.com/forums/showthread.php?253871-MCA-Bill-of-Sale
http://www.parkstonebayyachts.com/about-us/title-documents/
A reputable seller should also have a trail of insurance documents, license renewals, receipts for work done etc
Dominic's site also has useful information: http://rugbyboats.co.uk/Advice-for-boat-owners-on-the-inland-waterways
Have fun.