There are two ways of adding a different decoder.
I removed the three screws from the circuit board (having removed the decoder). Turning it over carefully so as not to remove the plugs (all black wires), you will see the brass coloured connector lugs. With a fine soldering iron carefully touch the iron and solder to each lug until the solder runs. ensure there is no overrun onto adjacent lugs.
Cut the 8 pin plug off your decoder, and remove a tiny piece of insulation, and prime each wire end with solder. Now carefully solder each wire colour for colour as it is in the decoder plug. Remember you are upside down.
This method leaves the decoder and plug in one piece in case you want to restore it later.
The easy quick method is to cut the wires on the existing decoder half way along , remove the 8 pin plug from the new decoder, strip and tin both sets of wire ends, and solder together wire colour to wire colour, using shrink tube to insulate. You will be left with a green and purple wire which are not needed.
I found that after fitting my Lenz decoder L10331 the usual growl on the motor at slow speed disappeared.
This was also found by at least four other people who changed decoders after this was reported by myself and another person on the Hornby Forum.
Decoders used were Lenz, Digitrax, and TCS.
This discussion can be read on the Hornby Forum - DCC section - View all discussions - Smooth Pendolino
For information, I use a Prodigy Advance 2 controller.
AlanB