A 769 on the other hand a) has a tightlock coupler, where the pneumatic connections are in the box underneath the coupler, requiring a secondary box to be attached to transfer air and b) have an electropneumatic brake system, requiring the loco to be able to send the right signals (being an old unit - voltages on select wires) to actuate the brakes on the unit, requiring that box to go underneath the mechanical coupler again, but also the right thing on the loco itself to do the talking, which AFAIK only the ROG/Europheonix locos actually have.
This picture of a 321 being hauled to Newport managed to get a reasonable closeup of the coupling between the unit & the loco, you can see the aforementioned box connected to the 321 with (what appears to be) main reservoir pipe on the left with the yellow star valve, and the control cable disappearing off on the other side of the box and into the loco's nose (level with the 321's headlights)
All this supposes of course that there isn't a brake issue on the 769 itself!