I've now fitted a DCC decoder, and in the end managed quite easily to fit a standard Lenz HO Gold inside the loco body.
Remove the body (very tight fit) by removing the second screw from the rear and the front screw (immediately to the rear of the front pony truck) to reveal two distinct spaces in the boiler/firebox area.
The rear space is occupied by the motor & gears, and the DCC socket is located in the front one. Between the two are two cast weights, each held in place by a single screw. Remove the front screw and allow that weight to slide out of the boiler towards the smokebox door. The ballast space provides just enough room for an HO Gold decoder.
I created further extra space by removing all of the function wires from the decoder, and inserted the white plug first (with remaining wires taped to it) into the space created by removing the weight. The decoder was of course simply plugged into the DCC socket and the yellow capacitor removed from the PCB. Tidy up the wires before replacing the body and job done - beautiful runner, and looks good too.