No there have been no alterations of the sort you mention s has received no amendments And just because an Act is  years old doesnt make it out of date Also the reduction for a guilty plea is not applied mathematically post sentencing See eg sd Criminal Justice Act  The process begins with the court deciding the sentence that should be imposed for the offences then selects the amount of the reduction by reference to the sliding scale then applies the reduction and finally pronounces sentence in court explaining what reductions were applied