[K03NT3S31N]

I don't know how this will help you, but I have the following idea for your current situation:

When you got the 1300£ GPU, your old 300£ card was also removed and lying around. After realizing that the 1300£ GPU "wont work", ...
a) you accidentally packed the WRONG GPU (distracted from work, stress, family problems or something like that)
b) a roommate prepared the return package FOR YOU because you were at work. Your roommate accidentally packed THE WRONG GPU.