"… The Definitive Guideline provides no guidance as to the length of disqualification and so it is important to bear in mind, first, the risk represented by the offender is reflected by the level of culpability which attaches to his driving, and, secondly, the main purpose of disqualification is forward looking and preventive rather than backward looking. In that regard the [offender’s] previous unblemished driving record is clearly an important factor as is the absence of aggravating factors such as speed."