Had a good read of the technical manual today so time to bore everyone.
Apparently ASR was available as an option up until June 2003. After that date it’s ESP or nothing which would pretty much explain why I’ve only got ASR.
It would seem ASR is an enhancement of ABS and ESP is a further enhancement of ASR. It would be wrong to think of them as separate discrete systems and this is why it’s impossible to have ESP without ASR (and for that matter without ABS)
ASR and MSR are a lot more sophisticated than I first thought. Although ASR only controls engine torque and breaking on the driving wheels it actually uses all the information available from all 4 wheels. For instance it uses relative rear wheel rotation to work out how fast you’re cornering and responds to lack of traction accordingly.
ESP goes one step further by utilizing information which can’t be obtained from purely wheel rotation. It uses an extra sensor to detect the actual forces acting on the car and in particular rotational forces about the cars axis. A 2nd sensor on the steering rack provides information about what the driver’s doing.
Since every ESP system must include ASR it follows that a switch to disable ASR must be included. Otherwise, you wouldn’t be able to use snow chains or drive in deep mud

or snow.
From what I can gleam upgrading from ASR to ESP would be quite a challenge as not only would you need to install the extra sensors but the control unit would need to be changed too.
I’ve not been able to find any information about why some ESP systems use a single display light and others use two - so that still remains a mystery?
Come on Techies - somebody must know?