This did it. It was Just the 4 clips. Theres no clip in the middle.
My 2TB hard drive (WD20EURX) I put in from my SKY HD box was double the thickness of the 500GB drive in the Humax but just about managed to fit.
It formatted fine using the Humax settings and works great.
Also for anyone thinking of changing the hard drive, 2 of the hard drive screws have a warranty seal so a hard drive change will void warranty.
Thanks for you help guys.