Well finally had time so here goes,
The bits! One loco, one LoksoundXL and a dcc plug.
remove the two screws at the front as per the instructions
and just the outer two on the metal bracket at the back.
You may find the cylinder top covers pop off too and you end up like this.
Now this proves they've used the original Lyn mouldings design even if it is a new mould as the old attaching spigots are still there, circled in red here, and they limit the space in here too. I suppose you could chop them out but that will be pretty fiddly. I measured it all up and the Bachmann circuit board goes right up to the top too so no room above this for the decoder.
Here's the circuit board with the DCC socket arrowed. If you buy a plain decoder simply remove the blanking plug and plug it in here. Note also where they've chopped off the old mounting lugs where the body attached on the original model.
Now you could drop a small decoder in the space arrowed but it's not big enough for a G scale sound decoder so I assume they were intending to use a HO/OO gauge one??? Tabs are provided on the circuit board to solder the speaker outputs from the decoder to to use the built in speaker.
Note I have made up a decoder lead using the plugs I already had but you can buy these ready to just screw into the decoder terminals.
I used the loksound XL though so I needed more space. I removed the cab as the instructions. Then I chose to drill a hole halfway up the firebox side, (to avoid the pocket for the decoder seen in the picture above), and use the cab floor as there is loads of room.
Simply attach the relevant leads to the correct terminals as per the ESU instructions, I haven't used the NMRA standard colour code so don't use my coloured wire as a guide

You can cover the decoder with black tape to hide it.
I also chose to use the supplied ESU speaker so I removed the installed one and chopped off the surround with side cutters. The ESU one is then simply attached with some very strong outdoor double sided tape from Maplins that also prevents vibration. I also added a piece on the back so if it touches the cab I won't get vibration.