Since this is a byelaw 18 (and 23) prosecution, the question of short-faring is not relevant.
In relation to byelaw 18, and again assuming that Mossley had no purchase facilities at the time of travel, the prosecution must essentially prove, beyond reasonable doubt, that the OP boarded at Ashton.
The OP will presumably present sworn direct testimony, under oath, that he boarded at Mossley, perhaps supported by indirect circumstantial evidence such as evidence of car parking, past journeys, and local residence.
Northern will presumably present the RPI's sworn testimony that the OP said he boarded at Ashton. This is merely second-hand evidence, since the RPI did not see the OP board. Furthermore, the RPI could well have misheard, and indeed it would be possible, from the point of view of the bench, that the OP boarded at Mossley but mistakenly (or otherwise!) claimed to have boarded at Ashton (perhaps attempting to short-fare?), which would still leave him not guilty of the specific byelaw 18 offence as charged.
I find it difficult to see how a properly directed bench could be sure beyond reasonable doubt that the OP boarded at Ashton. Even if they think he asked from a ticket from Ashton, this still does not prove he boarded at Ashton. He/ she may have boarded at Mossley and attempted to short-fare, which would leave him guilty of other, more serious offences, but not the specific byelaw 18 offence of which he/ she is charged.
In my opinion, the OP is more likely to be found guilty of the (less serious) byelaw 23 offence than the byelaw 18 offence. If the OP is considering pleading guilty to only one charge, then, IMO, it should be 'NG' to byelaw 18 and 'G' to byelaw 23.
However, if the OP, upon seeing his/ her solicitor to make the SD, was able to get advice as well, he/ she should probably follow that advice, which may well include not making posts on a public internet forum.
