The 'traffic warden' (who, these days, is more likely to be a 'parking enforcement officer' employed by the council) is correct.
A similar situation happened in Sheffield where SWFC supported had regularly parked 'inside' double yellow lines on Herries Road South for many years. That was until the day that the police decided to enforce the regulations and around 100 drivers found that their cars had been 'ticketed'. There was a a great deal of fuss made about it (with substantial coverage in the local media) but the police wouldn't back down and the drivers had to pay up.
Chris