Just a quick update (posting during work hours) - letter received this morning from South West Trains, £80 fine was issued. So to those who are worried about fare evasion - two things:
1. Do not worry about court action at first. I know it is case by case, but it appears fines are the first penalty. At least for South West Trains anyway.
2. Please buy a ticket from the conductor on the train asap if not before, like others have said. I am not the right person to give this advice, but I made this mistake not because I had intentions to evade, but because I was not aware of the rules.