My "standard" decoder is the Lenz Silver Direct. However, and extremely annoyingly for a loco of this size, obstructions in the chassis prevented this. Instead, I implemented the good old Plan B of the Lenz Silver decoder.
First task as always is how to remove the body shell. On this model, three screws under each bogie did the job - two of the three are pictured here under the bogie. Once these are all removed, the shell can be removed.
Here's the bare naked loco. Reminicent of a Heljan loco. First things first, snip off that capacitor labelled "C1". It'll only cause problems on a DCC layout. Next, remove the little plug on the DCC socket.
Once that's done, simply plug the decoder in to the DCC socket, taking care to ensure the orange wire is at the Pin 1 end. Apply the supplied double-sided sticky pad to the circuit board and attach the decoder as shown.