Railway fare evasion is contrary to the Regulation of Railways Act 1889 (yes, eighteen eighty-nine). It is punishable by way of a fine of up to £1,000 or three month’s custody, though magistrates’ sentencing guidelines suggest a fine of about a week’s net income (reduced by a third for a guilty plea) for first time offenders.
It is a “summary” offence (that is, it can only be dealt with in the magistrates’ court). If you decide not to pay all or part of the penalty the train company has levied they will no doubt launch a prosecution. If you are convicted you will have a criminal record, you will be liable to a fine as I suggested above, you will pay court costs (up to £350 if you plead not guilty) and a £15 Victim Surcharge.
The situation that No Mercy describes is completely different and is covered by civil law, so a comparison is not valid. Contrary to No Mercy’s advice about paperwork, it is not necessary to issue documents or get you to sign anything to prove that the incident occurred or that the correct procedures were followed. The ticket inspector can give oral evidence to show the court what happened. So far from having “everything on your side” I would suggest that the only possible defence you have is along the lines that you were not warned in time that a prosecution would follow. If you were not warned at the time of the incident you should have been told within two weeks. (If the warning was by post it must be posted so as to arrive within two weeks in normal circumstances).
You may, however, find this difficult and in any case it is not an absolute defence. The court will consider whether, in all the circumstances, the prosecution was properly brought.
My advice – pay up, thus avoiding a heftier fine and costs and a criminal conviction.