After a frustrating month of hard resets, factory resets, sending reports in (as requested) of separate attempts to connect to the internet, replacing the sim, being told that 'they will come back to me' - I have finally given up!.
If you read other comments on the web you will find hundreds of new Plusnet customers who have been unable to access data/get an internet connection. The staff member who finally gave me my pac code after 3 x 15 minute attempts to get through did admit 'they had a couple of others'!!!!! with the same problem.
Guess I will now have to crawl back to EE with my tail between my legs - If you learn from my bad experience it would have been worthwhile:-(