This is VERY sharp practice by Islington, but typical for London councils, I'm afraid. A good job you took pictures of the sign before and after.
For now, all you need do is submit an informal challenge stating that when you parked there was nothing to indicate the extent and times/dates of the suspension. As there was no information about the suspension the PCN is clearly void and must be cancelled. What should have happened is the suspension details be glued onto the yellow sign, then the sign put up.