Now, the latest challenge, thanks to the shop at the Chasewater railway and my mouth.  Fit a DCC decoder into the Bachman wickham trolley.  It has to be invisible and having dismantled the offending item, I decided it would be better under the floor of the trolley (saved interfering with the motor on the trailer).  I also decided to keep the electrics as original as possible, to save issues.  So...

First, modify the PCB in the trolley chassis, removing the capacitors and chokes, plus drilling out the holes in the PCB, to make it easier to connect the decoder wires.

Next, I attacked the chassis with a burr in the Dremel flexi-drive.  The idea was remove enough material to ensure the decoder (Zimo Mx616R) fits with its heat shrink intact and give a bit of room for the wires.  Straightforward job, slow and steady sees it done.

Like so.

Hopefully, having trial fitted the PCB, all that remains is to solder the four wires to the connections on the PCB and reassemble.  Stay tuned for part 2, to see if it worked.

Owain