Update: I have received a response from TPE. The long and short of the response was that nothing negates my liability in this matter. This i understand. However they considered the mitigation and the circumstances and so have offered to settle the matter out of court provided i pay £10 of the fare 'avoided' and £50 towards costs to date.
Although I didn't actually avoid paying the fare as suggested, I am content with this resolution, as I know this could've been a lot worse, and they didn't actually need to consider my circumstances. I have since paid the £60, and know to avoid making this mistake again!
Thank you all for your guidance, It is very much appreciated.