I've found this thread as I (and several others on my train) was charged a penalty fare at Stansted on Friday, having changed from the tube the overground at Tottenham Hale. I have no idea if I'll be successful on appeal (and can deal if not) but I think this is a really poor business practice by Greater Anglia which needs to be challenged.
The fact that there are lines of ticket officers who spring immediately to issuing a £50 fine just seems bad faith (the officer said he had no discretion, which I am surprised about). It seems to me to be taking advantage of a confusing ticketing system and a vulnerable customer base - by virtue of the fact that many will be international travelers and unfamiliar with the UK rail system/not English speakers. It sounds like I probably did miss signs at Tottenham Hale - I didn't really give it a second thought as I am used to travelling from Gatwick, where you have been able to use contactless for several years. Regardless, it should be obvious that I did not have an intention to evade the fare as I had an 'uncompleted' contactless journey.
I see London Travelwatch did release figures on fines back in 2019, so this has long been a known issue and nothing has changed. I have written to them requesting updated figures and to Greater Anglia asking what they have done to address.
Welcome to the forum!
I understand your situation is frustrating, and why it does seem like an unfair penalisation. I imagine the comment about no discretion is that everyone arriving in this situation will have the same backstory - in a rush for the train, didn't realise etc - as I expect they would have the discretion if there was a compelling situation.
For what it's worth in terms of addressing the situation, efforts have been made recently at Liverpool Street to stop people tapping in for the Stansted Express including the creation of 'Stansted Express Ambassadors' (I think this is their title) who are stopping as many as they can boarding the train to check their tickets. The barriers, or at least at Liv St Platforms 1-3, have stickers over the oyster card readers saying not to tap in for Stansted. Sounds like this initiative has not extended to Tottenham Hale - if it hasn't, it should be considered.
If you would like specific advice, please do feel free to start a new thread in this section (Disputes & Prosecutions) - although you have done all you can at this moment. Without preempting the outcome, it is likely that the appeal would only be successful on a technicality e.g. the absence of appropriate signage. I'm not sure the situation at Tottenham Hale.