Weird one here, hopefully someone has come across a solution!

A solicitors firm want me to start shooting their CPD lecture series to host them online for viewing later. However, they need to be able to verify that the person has actually watched teh video (as much as possible) - for example a code is generated when the video finishes that they have to input - or something along those lines! Any ideas what could work or some other crazy solution!

Not weird at all - this is commonplace with online training, for example. There are enterprise level solutions that allow you to track total time viewed by IP address and other metrics - I have experience with the MediaSite platform. Of course, with enterprise level solutions come enterprise level pricing...