Actually, its a mess:
If you are referring to a "Toastrack" (original Sinclair 128) model, the cable is nearly identical.
A Toastrack video cable can be used for the QL if you unsolder pin 3 (the Toastrack's "BRIGHT" output) which carries monochrome composite on the QL. In most cases you wont even have to do that.
Most Spanish Spectrum 128s might have an different monitor plug pinout, cables for this computer are usable on the QL, but will only provide a monochrome picture. (The Spanish 128 has no composite PAL on pin 1.)
If you have a cable for the Amstrad Spectrum +2, that should be identical to the original British Toastrack cable.
If you have a cable for the +2A, or +3, that's even more different, these computers have no composite (RGB only) at all, but provide some potentially dangerous (for your monitor) 12V lines.
Detailed information can be found here:
http://www.fruitcake.plus.com/Sinclair/ ... TCable.htm
This page has all mentioned wirings for the 128, the +2, +2A/+3, and the QL and their differences if you search around a bit.
Tobias