Spark Plug Socket

Little help here, rookie mistake on a service on the car, got hold of me spark plug socket went to undo them, socket doesn't fit down to the spark plug due to the head getting in the way.

Therefore i need a slim walled spark plug socket, but cannot seem to find one; would need a 21mm spark plug socket; but can only find 14mm thin walled. Anyone know where i can get hold of one?

Or are spark plug sockets spec'd to thread size … making the 14mm correct? Cheers anyone.