Ok fella ive sorted it, after much testing i have found a way to fit this decoder to this train without any mechanical modifying, so for anyone out there with this train and this decoder this is how it fits, hope this is helps.
De-solder the factory 6 pin socket, insert from underneath (the decoder fits in upside down) and solder the decoders 6 pins directly to the PCB, this frees up 6mm, as the decoder will never have to come out of the loco anyway, i think this is the best option for reliability.
Route the speaker wire under the pcb and fit the speaker the opposite end.
Everything is in tact, no mods of the chassis needed.
One of the tiny split chassis power screws flew somewhere so be careful, lol, gotta find another screw now from somewhere!
Anyway all done, sounds beautiful.
Now you have a spare 6pin micro male and female socket to make your own cab light power connection (from supplied dc plug)