Thanks for you detailed reply which helped me a lot in sorting the problem. After reading your reply, I pondered as to whether the plugs and leads were replaced prior to, or after the problem started and to be quite honest, I wasn't sure.
Pulled the plugs and put the old ones back in, after cleaning and gaping. Problem solved, or at least it hasn't returned after 100 miles. Nasty error codes vanished like magic. Not sure if it was a faulty plug, or the Champion plugs that were recommended to me just weren't compatible with the vehicle. I will replace with new NGK in he next day or so.
Thanks again for your help. I'm sure that I will return at some point in the future.