“The fact that the defendant owes the claimant a duty to take reasonable care in carrying out its (the defendant’s) activities does not mean that the duty extends to every kind of harm which might be suffered by the claimant as a result of the breach of that duty. … The scope of the duty of care assumed by a professional adviser is governed by the purpose of the duty, judged on an objective basis by reference to the reason why the advice is being given (and, as is often the position, including in the present case paid for … in the case of negligent advice given by a professional adviser one looks to see what risk the duty was supposed to guard against and then looks to see whether the loss suffered represented the fruition of that risk.”