I have posted before about this same problem. Occasionally, whether hot or cold I have problems with engaging reverse on my 147 2 litre selespeed. Putting into 1st and then reverse usually works, but sometimes I have to actually move forwards a few inches and then try reverse. This always works. I also often get the Selespeed System Failure warning coming up either when I am parked and about to pull away or during normal driving (in city mode). It is annoying but I am living with it for now. The agents are as usual baffled and it of course never happens to them! They have re-adjusted the hydraulics several times, and changed the brake pedal switch to a supposedly newer "4 wire" version. They are now ordering a new hydraulic system. Failing that they said they will fit a new gearbox. Sounds radical but I am determined to get this sorted out before my 3 year warranty expires.
Try this when the problem happens:
When you try and engage reverse but it stays in neutral. Press the accelerator, does it rev freely or not? Mine feels like their is resistance, almost as if it is in gear with brakes applied.
This may be caused by the hydraulic actuator not fully engaging the gear? Hence the change of hydraulics next. I will let you know.
I have some doubts and still think it is a software bug but cannot get any answers here in Cape Town. Mine is almost a year old and maybe has older software installed. Anyone out there know about any software upgrades?