GA are requesting £120.40... £100 is penalty fare, £20.40 is unpaid fare. Where is £20.40 fare from? I cannot find any stations where either CDS or SDS to SSD is £20.40.
London Terminals to Stansted Airport SOS minus the 4.60 paid? It's a settlement offer and not a penalty fare, so they can realistically ask whatever they'd like.
Breakdown of the settlement offer would be London Terminals to Stansted Airport SOS (25 pounds) minus the fare paid (4.60 pounds), plus a 100 pound admin fee.
I would suggest that speculating any more on the legitimacy of the ticket is unhelpful and this thread should be locked until the OP responds to answer some things absolutely truthfully (we aren't here to take sides):
1) Which retailer was used to purchase this ticket?
2) Where exactly was this ticket collected?
3) What time was this ticket collected?
4) What time were you stopped by the revenue protection inspector?
It's very obvious that this ticket has either been issued incorrectly (albeit not invalidly) by a retailer, or that it has been forged.
Suggesting that OP has forged this ticket is a wild accusation, and the company clearly don't suspect this as they would not be offering a settlement so readily.
If, however, the ticket has been forged, then I would suggest that OP pays the settlement immediately and does not say anything further to potentially self incriminate, and obviously ceases any of this behaviour.