From what I have seen c2c have a tendency to send out letters after 2 weeks I have also seen the first letter usually appears to be an instant settlement offer
Well 5 days is not 2 weeks, it's not even one week - but can you point us to where you have seen this info?
Or have you had letters from them before yourself about incorrect tickets? (ie do you have a track record of doing this)
C2C do not feature very often on this forum, so please share where you are getting that info from so we can potentially have a look at it too.
but with my case being so high value I'm worried they may skip all the previous options and move straight to court.
In my view of watching this forum long term, It's not generally the volume of the fares owed that influence the decision on whether prosecution follows or not - it's potentially the motivation deliberate or otherwise to not pay the correct fare.
Now you
claim this was all a mistake about selecting the wrong railcard - so if that is the case you are not evading deliberately and you will need to convince them of that
BUT
if you
actually have been evading deliberately by selecting the 16-17 Saver discount that might put things in a different light and you might have to work harder to persuade them not to prosecute.
It would help if you could be honest about it here at least - as then people can give the best advice to you.
Having said that - people who do deliberately evade when caught often still achieve out of court settlements.
As mentioned above you have paid the wrong fare (intentionally or unintentionally) for a long period and thus the sum of money they may want from you to settle out of court is going to be quite large potentially.
So at this stage instead of worrying if they will take you to court or not, I'd focus your thinking on how you can save up or borrow from trusted family or friends (definitely not dodgy money lenders) the kind of money this might involve, as mentioned up thread. This is probably an important step you cane take now.