The photo shows you parked partly off the carriageway and also next to a single yellow line, yet you say you saw no prscribed hours on the street. This means it is virtually certain you were parked within a Controlled Parking Zone, which controls the single yellow lines by having signs at the zone entrances with the restricted hours thereon.
And indeed you were in a CPZ, with the sign on Walmer Terrace, shortly before you turned into Southport Road:-
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CPZs do not have to have local signs giving the restricted hours for single yellow lines, the entry signs are sufficient. My only comment is that there is only a single sign and it is on the opposite side of the road. There really should be two signs for a CPZ entrance, but the council will no doubt argue one is sufficient, thus forcing you to either pay the discount or take them to London Tribunals, where you must risk the full PCN penalty.
HOwever, if it were me, I' submit an informa challenge stating that you are now aware of the sign, but failed to notice it on the day the PCN was served, and it is very likely this was due to the presence of only a single sign on the opposite side of the road. Post up their response when you receive it.
However, I am puzzled why the CEO served a PCN for parking in restricted hours, when he could have served one based on the very long-standing offence of parking off-carriageway. This has been an offence in London since the 70s and requires no signs or lines, and is very difficult to refute.
