Hmm, so you left the car parked for 11 days without attention or use. This is a period that would allow them at least 7 days to put up the sign and to start whatever work was intended. A bit risky leaving the car such a long time especially in London. In fact you're lucky not to have been towed. A picture of the suspension sign would be useful, if you can get it.
Why did you leave the car so long, holidays/work abroad maybe ? The main point is your car was legally parked before they put up the suspension sign, so should be on the record they keep when putting up the sign. So your reps need to emphasise this and also state you are a resident. They should then cancel the PCN, but probably won't so you'd have to take them all the way to adjudication.
Other thing to ask is - do you need a resident permit in order to park in the bay where you parked.
Anyway, see what the others ask or say.
