I've done the heater conversion.

[Saga Lout]

As the title says, I've converted a single manual control into a dual zone heater. The job wasn't hard at all but there are some restrictions on who can do it, to achieve the end result you need a manual heater box, a digital box and a spare knob and cable, plus the actuator linkage that opens the flap from a manual one, those are the three things needed to do it.
To do the job, open up both heater boxes and put the parts to each at the side and don't mix them up, after taking off the clips that hold the boxes together don't miss the hidden screw near the fan housing. To start, get the two halves from the manual heater box, put the half flaps into one side, then put the divider plate in the middle and make sure it clips in properly at the top, you can then fit the other half of the box and that part is job done, clip it back together.
The left side linkage will go right back on as it came off, make sure to use the long plastic actuator that came off it. If you've done the mod on the control panel you'll have a spare linkage and cable, this has to be reversed, I warmed up the metal and bent it exactly opposite to what it was, I then drilled out the pin and soldered in a new one. The linkage is almost ready to fit now but you have to put in a dog leg bend as the plastic part is a reversed one from a left side. I drilled a small locating hole in the axle of the flap behind the one at the front, it allows the clip to pop in. That's it, job done and very rewarding.