(Just to add regarding the date, the notice was completed in the presence of the person it was handed to, so it doesn't seem unreasonable that an obvious mistake that is self-evident to both sides could be corrected later.)
I've taken a good look at Paddington station and I would suggest you lodge an appeal on the grounds that the requirements for notices are not met (quoting the relevant bits of the S.I. including 16(3)(a)). In an appeal against a Penalty Fare, it is for the TOC to show the contrary (i.e. you do not have to prove the posters are missing/non-compliant but rather they have to prove they are present), and having walked round Paddington Station, I struggle to see how they could possibly do this. Posters I remember before have been removed and one obvious location for a Penalty Fares warning poster at one entrance even instead simply has a picture of Paddington Bear!