Having penalty fares just for mistakes is a bit unfriendly, though. If it's possible to weed out those deliberately evading the fare from those who are not, it'd be better just to charge the required fare to those who are not.
The trouble is that differentiating is difficult, which is why I would decriminalise fare dodging in the manner of parking, and simply set a high penalty fare along similar lines. Something like £100, reduced to £50 if paid on the spot or within N days, might be about right, perhaps removing the discount for a second offence within one month. You would then pay the correct (discounted if appropriate) fare on top in addition. You then grip often enough to ensure there is no financial loss from fare dodging, and leave it at that.
If some people would rather pay £100 when caught once a week then £80 for a season ticket, say, why should I mind?
But for a PF scheme like that to work it would need to be administered differently. Sham PF schemes like LM's where the guard can and does sell normal tickets are pointless. Guards need to be able to issue PFs and only PFs, like they can in Switzerland (though perhaps you'd need to send an additional security guard around with them on particular trains). You pay before you board, or you WILL be PFed, is the impression you need to give to people. The only exception being a verifiable closed ticket office AND broken PERTIS machine.
Neil