I guess I will find out, then again £50 is a small price to pay when the alternative could be a trip to the local A&E.
I think you should make an Appeal, but you need to be prepared to do that at each 3 appeal stages. You should perhaps get your appeal wording checked here before you submit it. You could draft your wording whilst waiting for the site to allow you to submit it.
The clock is stopped on the prompt payment discount deadline whilst Appeals are considered.
Unfortunately however, Appeals have to be based on the fact that the Penalty fare was incorrectly issued as per the legislation, and I doubt the legislation will specify an exemption for using the ticket machine because you felt unsafe to use it - though you very much have my sympathy.
With hindsight I suspect you would have been in a better position if after boarding you had actively gone to find he train guard, explained the situation, and actively asked them to sell you a ticket for the reasons concerned. A guard on the train will probably know stations where youths hang out like this, may have even seen them and might be happy to sell you that. the remote Appeals processing company will, I fear, care little about things like that and simply focus on the beaurocratic process of whether you were issued with one correctly or not.
On a separate matter it may be worth contacting Northern Customer services about youths hanging around and intimidating people at the station, and indeed report it to BTP on the 'see it say it sorted' text number. You would at least have a record of that to add to your appeal.
Could be helpful for next time as of course some of that is water under the bridge now....
tho nothing to stop you reporting it in general now since the incident was yesterday so it's still fresh.
I would certainly be doing this if you felt the behaviour was intimidating enough to stop legitimate passengers from using the machine, which it was in your case.
As I say, you have my sympathy.