“From the 1986 Act onwards, the courts have been required to reinforce confiscation orders by the imposition of a term of imprisonment to be served in default of payment. But it has been recognised that a defendant may lack the means to pay a sum equal to the aggregate of the payments or rewards he has received, or the value of the property or pecuniary advantages he has obtained. It has also been recognised that it would be unjust to imprison a defendant for failure to pay a sum which he cannot pay. Thus provision has been made for assessing the means available to a defendant and, if that yields a figure smaller than that of his aggregate benefit, making a confiscation in the former, not the latter sum.”