It's at the council's discretion whether they consider your representations or not, so you need to make it easy for them to reject rather than disregard:
Dear Bristol City Council,
I am making representations out of time because I have been abroad and have only just returned to the UK, I attach proof of travel.
I have not been able to make representations against the PCN due to being out of the country, so I have now lost the opportunity to pay at the discounted rate and the council website shows the amount owed is £189. I must have entered the zone accidentally for a few minutes so I would ask if you would be willing to cancel the PCN on this occasion.
If not, I would ask that you at least reinstate the 50% discount.
Yours faithfully,
Once you have a Notice of Rejection, we can appeal to the tribunal on more substantive points.