Thanks Richard,
I actually managed to sort it without changing anything other than removing the solid plastic behind the grill. It is a pre sound version, probably about 10 years old but it is DCC ready. By having the chip so that the dome piece faced down and into the slot above the flywheel and separating the wires so half are routed each side of the chip and not over it, the body slid all the way down to where it should be.