My bad - misread the OP's post saw "speed awareness course" and missed that he was never offered one.
2 NIPs - 1 for seat belt offence, course taken, matter disposed of. Other offence speeding, course offer expected, SJPN received. Discrepancy between OP's recollection of alleged speed on NIP and speed in witness statement. If the discrepancy is resolved in the OP's favour, he should expect to be sentenced at an equivalent level to the fixed penalty he would have expected to be offered.
Broadly speaking, speeding is the nature of the alleged offence. If the allegation was 35 in a 30 and the OP received a NIP for 103 in a 30, it might be arguable that that was an entirely different kettle of fish, but not 88 or 69 in a 60. The offence is speeding, the number merely concerns the sentence.
If the OP is certain that the NIP said 69 and that that is as fast as he would have been going, the solution would be a Newton plea - plead guilty to the offence but disputing the material alleged facts.
Unlike being stopped at the time, where a warning for any offence to which s. 1 RTOA 1988 covers all offences that require a NIP, a postal NIP must specify the nature, time and place of the alleged offence. *If* the only speeding evidence is from the location to which the seat belt NIP applied, there might be a vague/wrong locus argument - but this is verging on idle speculation