Parked on payment and accused of blocking emergency route: should I contest £30 parking ticket?

I have received a fixed penalty notice (non-endorsable) for a parking offence. The offence category is given as N2007, and in the brief written explanation of the offence the following appears: 'Parked in such a manner, emergency vehicle cannot pass'. I do not believe that my car was causing such an obstruction, and although my car was parked half on the pavement, I did so precisely to ensure that plenty of space was left in the road. I could accept if the ticket had been issued for parking partly on the pavement, but as this is not the reason cited on the ticket, I therefore have several queries about this fixed penalty notice: 1. I was under the impression that white non-endorsable fixed penalty notices such as this had to be handed to the driver in person. This was left on my windscreen. 2. What is offence category N2007? 3. On what grounds has the judgement concerning the obstruction to an emergency vehicle been made? Would there have had to have been an actual vehicle attempting to pass? If so, how big does it need to be to merit a ticket? (I'm sure an ambulance or regular-sized fire engine would have been able to pass, but probably not a large turntable fire appliance - but then that is surely the case on many narrow streets. If, on the other hand, this is a hypothetical judgement (i.e. the issuing officer has decided that my car would be obstructing an emergency vehicle if one were to attempt to pass), then what are the criteria used - is there, for example, a minimum space one can leave? It seems rather vague and unsatisfactory to me. Is this worth contesting on any grounds, or is it a case of pay it and forget about it?