Thanks for all your help everyone, I'm much more at ease with changing the hard drive now.

I guess the remaining question now is to find a hard drive to replace it with. I'm looking to go as far as 2TB but not solid state as that's too rich for my blood.

Currently I'm looking at a Samsung M9T 2 TB 9.5 mm SATA 2.5 inch PS4 Compatible Internal Hard Drive. Currently on Amazon for £82.

Does anyone know if this is good or can recommend another one? I'm all ears?