Ok I figured it out.
It's kinda strange but after writing the post code in I just pressed Enter.
Then I noticed the last letter of the post code didn't get capitalised like the rest and that was throwing up the error.
If I pressed the GO button instead of just pressing Enter it capitalises it and carries on. Also if I capitalise the last letter myself and press Enter that works too.
I guess I'm the only lazy one that just hit enter instead of pressing the button