I'm sure that the signs overide the lines in 'disputes' or claims that there were no lines visible - i.e. if the lines get covered in snow or are temporarily 'removed' such as road work for example, the barstewards can still do you.
But I suggest you go back and check exactly the spot you parked to see if it were double reds which is no parking 24 hours a day, 365 days a year.
If not, and it was a single red, and looking at the number of cars parked along the kerb that suggests that might be the case and you know the time was after 7pm, then you were entitled to park there. But as I say, check very carefully the spot you parked - around London they will happily do you for just a few millemetres 'over the line' (or double line in your case) albeit the rest of the car was lawfully parked.
Hope this helps