Hey Andy,
I was looking into the same thing when I owned a 2.5DT also.
The cruise control on the diesel is a fly by wire system and electronically controlled by the ECU so there are no bellows to fit.
As far as I remember, it needs a cruise control inverter or converter (can't remember which), activation through Testbook, a cruise master switch and the steering wheel controls.
I problem I hit upon was that on the existing loom, there are no connections for the master switch, the converter and the steering wheel switches.
So it would need a new wiring loom fitted from a DSE model to work.
The wiring diagram and more details are in Rave.
The work could all be done by yourself if you have access to Faultmate, Testbook or Rovacom.
I never got around to finishing the project as I traded her in for a 4.6 HSE.
Hopefully that info might get you started.
Ray.