If you want to fight this, you first have to submit an informal challenge against the PCN. Then, if you're unhappy with their reply, make a decision whether to pay the discounted amount or take the matter further. The alternative is to just pay the discounted amount now, but doing this before any representations are rejected is not recommended. The discounted amount will be re-offered if you submit your reps to arrive with them before the discount period expires, (see your PCN).
Try something like this: -
Dear Sirs
Re postal PCN <number> dated <dd/mm/yyyy> for alleged contravention on <dd/mm/yyyy> at Leytonstone Road
I deny liability for the above PCN penalty on the grounds that the contravention did not occur due to very poor signing of the Event Days restriction.
When I received the PCN, I was puzzled about why it had been served as I had passed no sign about event day dates on my journey from Mile End. I therefore took some advice, and it was pointed out to me that there was an entry sign to the Zone on the main road just after Marshgate Lane. A look at Google Street View showed an isolated single sign on this wide and busy dual-carriageway where the next event day date is displayed. I definitely did not see this sign, and can only surmise that a high sided vehicle such as a bus or van obscured my view when I drove past, or my attention was to the traffic lights only a few yards distant. I find it quite incredible that the council regard this single sign as adequate to convey the event day information, as it is not located at any junction or location where a motorist might expect to see a sign regarding parking.
In view of this failure to follow the regulations for placement of signs advising of parking and traffic restrictions, I request that the PCN be immediately cancelled, and the council cease enforcement until signage of event days is considerable improved.
Yours faithfully
