“It is, I accept, foreseeable that in these situations the suspect may be put to expense, or may conceivably suffer some other economic loss, which might have been avoided had a more careful investigation established his innocence at some earlier stage. However, any suggestion that there should be liability in negligence in such circumstances runs up against the formidable obstacles in the way of liability in negligence for purely economic loss. Where no action for malicious prosecution would lie, it would be strange indeed if an acquitted defendant could recover damages for negligent investigation. Finally, all other considerations apart, it would plainly be contrary to public policy, in my opinion, to prejudice the fearless and efficient discharge by police officers of their vitally important public duty of investigating crime by requiring them to act under the shadow of a potential action for damages for negligence by the suspect.”