Not sure about the serial number either, although if it came from a Log Book, it's almost certainly correct. Whitley MkV serials began the the N**** range, and ended in the LA*** range, but also included the Z**** range! The P range serials were mainly (but not exclusively) used for fighters.
It can always be difficult to pin down an individual code letter, as most entries in aircrew Log Books and Squadron ORBs showed either the serial number, the last three digits of the serial, or the individual code letter, very rarely both. Also, there would almost certainly have been, at some time, more than one aircraft with the same code letter, for example 'D', due to loss, replacement etc.
I've had at look at my references, and so far, can't match the serial to an individual aircraft. Might be a case of choosing a letter, perhaps an initial from Grandfather's name, and using that as the individual code.