They were following their sat-nav.
Oh dear, how many times do we see this on threads for in and around London. Unfortunately, appealing saying you were following satnav instructions is not an acceptable reason for cancellation of the PCN, because the motorist has a legal duty to look out for, and obey, any traffic signs imposing a restriction.
As there is a clear advance sign, plus the signs at the entrance to the restriction, I'm afraid there is little to go on for representations on the contravention itself, but there may be a 'technical' appeal argument based on council mismanagement of the enforcement process. These are unrelated to the actual contravention, and have won appeals at London Tribunals many times in the past 18 months.So wait a bit to see what is suggested, but don't miss any deadlines on the PCN.