Yes, as far as I am aware they do sometimes use partial index numbers. Quite how they determine which vehicle, from all the possibilities provided by the partial number, they choose to pursue, is anybody�s guess.
There are conflicting views concerning the right to see the photographs. Most camera partnerships allow you to view the photographic evidence they have by appointment before you decide whether to accept a FPO. You most certainly must have them provided to you if you plead not guilty in court.
The time to decide whether to adopt either of these courses, though, was at the early stages of this affair when you received the original NIP. If you believed then that there was a possibility that the vehicle was not yours, then that was the time to set about establishing the facts.
However, you have a problem now. As a result of taking the advice to ignore the request for the driver�s identity, the original speeding charge will now almost certainly have been dropped. You now face a Section 172 offence and the photographic evidence is no longer relevant. Only if you identify the driver as yourself and persuade the CPS to revert to the original speeding offence would it become relevant again. The CPS, for its part, however, will probably only revert to the speeding offence if you plead guilty. Otherwise they will simply continue with the S172 prosecution.
I think you must just sit tight and await further developments. Any prosecution will now almost certainly be under S172 (the time for beginning a speeding prosecution has elapsed anyway). I think your only possible escape route is to hope that it is either not pursued at all, or if it is, that you can persuade the court that it is �out of time�.
Hope this helps.