Sonic Models have just advised us that the 6 pin socket on PCB may be offset a little bit on some models. It may cause a difficulty to re-assemble the body shell after fitting the suggested decoder (Bachmann right-angle decoder 36-556RA) because the inner space is very tight on such a small loco.
After further checks of current 6 pin decoders, there are 3 version with a very small size, they are:
ZIMO 6 pin decoder direct MX616N
ESU 6 pin decoder direct 59827
One customer has reported that an alternative is to remove a small amount of plastic from the decoder socket so that the body can sit flat and enables the original suggested decoder to fit correctly.
Sonic Models apologises for any inconvenience that this may cause customers.