Can you clarify the address thing? Do I understand correctly that at the time you gave two addresses (one current and one previous) both of which may or may not be in the inspectors notebook? And that Northern used the wrong address in error?
My own thought (some legal training, but certainly not to the level of other forum members) is that if so, then they are now out of time. HOWEVER whether this is accepted (especially if no evidence to support that you gave your correct address) is another matter. Do you have the money to pay a solicitor to negotiate with Northern Rail on your behalf?
Other thoughts?
Provided they originally went to court within the time limits they should be fine. The OP's declaration that they did not recieve any notification of the case has effectively reset the clock.
IANAL