The new standard Hornby chassis is superb, free running and full of pulling power.
Smaller decoders can be mounted in the Loco. Mine has a Digitrax DN121 in the loco, with a Sleuthe No 22 smoke unit (for plastic bodied locomotives).

The basic chassis with the capacitor removed as always.

Join the yellow and white wires (neutral) To one side of the smoke unit. The blue (live) to the other. I retained the Hornby tender coupling on this loco. The smoke unit is rated at 140ma well within the 200ma output rating of the function output of the decoder.

Check and make sure the wires coming from the
Tender go to correct side of the decoder input wires IE the red and black wires. . The middle photo clearly shows the shrink tubing used to seal joints.
To check your connections place the loco on a spare piece of track “Unconnected to your layout or programming track”. Check both poles with a continuity tester to make sure the wires from the tender are not reversed.