The idea of the hidden hare is very good, and it would be in keeping with the way that the theme was treated at the time.
The problem, surely, is that 'four consecutive letters in a straight line' within the creature's profile could only lead to ARCH, at least if we want to go on believing that plain English is involved.
So what about the theory that those four letters are in the preamble? It would be very perverse of the setter to think he had to give a hidden indication of HARE in that way. Once you have the reference to Ampthill then either memory or the internet leads you straight to the bleedin' obvious.
And wouldn't it have been enough just to say 'four consecutive letters'? The addition of 'in a straight line' is only needed if the letters are in the grid.