No, all (except RML2760 which is not, officially, allocated to the 15) were refurblished by Marshall and the drivetrain was completely replaced with a Cummins B-series Euro 2 engine and an Allison gearbox - essentially the same as a Dart (hence they are known as Dartmasters).
SRM3 got the silver livery in 2002 (Golden Jubilee of the Queen's Coronation) that it used to have at the Silver Jubilee of the Queen's Coronation in 1977. The Brown/Grey livery of RM1933 is the centenary celebratary colour of the opening of Bow bus garage.