The BBC Master Compact (MC) computer has a separate unit (Disk Drive & Monitor Stand) that houses the power supply and 3?" floppy disk drive. This unit is not really necessary to use the Master Compact as any 5V 2A power supply with 2.1mm plug could be used (i.e. even a USB charger), and a Gotek can replace the floppy drive (apparently not much software came on 3?" disks for the BBC).

The MC uses a DB25 socket for the floppy interface. If you have a MC but it didn't come with the disk unit then you can make up your own floppy cable by either using a male DB25 plug with crimp connector and 34-way ribbon cable ... or build this simple board that uses a DB25 plug with solder cup connectors and re-use an existing 34-way floppy cable.

Some people prefer to make up a cable using the crimp connector, some people prefer to make up a simple board. Either way, the pin out is pretty simple - DB25 on left, floppy ribbon cable on right:

 1...INDEX.....8
 2.../DS0.....10
 3.../DS1.....12
 4.../DS2.....14
 5.../MOTOR...16
 6.../DIR.....18
 7.../SEEK....20
 8.../DATA....22
 9.../ENABLE..24
10.../TRK0....26
11.../WRPROT..28
12.../READ....30
13.../SIDE....32

(see also my Github page)

If you want to use a DB25 with crimp connector then do the following:

[1] Pins 14 to 25 on the DB25 and the odd-numbered pins on the ribbon cable are ground and thus should connect,

[2] Pin 8 on the ribbon cable connects to pin 1 on the DB25 - discard the first six ribbon conductors (1-6) and the last two (33 & 34)

If you want to build my board, just get a generic 2x17 pin header and male DB25 plug, solder them to the board, then connect with a standard 34-way IDC ribbon cable to the Gotek.