The length of the ban for failing to provide is on a scale divided into 3 parts, with escalating degrees of seriousness, leading to a ban of between 12-36 months. You can read about it here:
http://www.drinkdriving.org/drink_driving_sentencing_guidelines.php#failspecimen_drive
if you had provided a sample, the police would have had to prove "beyond reasonable doubt" that you had driven the vehicle. Because there was no sample provided, they only had to prove that they had reasonable grounds to suspect that you were, or had been the driver.... and they had this from 2 witnesses so the case was made out.
I have a pretty good knowledge of the law and police procedure in relation to breath tests as I have been involved with the criminal justice system for 30 years, followed by another 13 years directly involved with the delivery of Drink Drive Rehabilitation courses.
We have about 45 trainers who deliver these courses, and it is my jobs to keep them informed about changes in the law and about changes to procedures. That is why I can answer your queries and am happy to do so. I do this in my own time, having seen many people in your position and want to try to smooth the way through a distressing time in a persons life. I do not condone drink driving, but the actual punishment is enough, without the heartache people go through when they do not know what to do next.
You will see from my logo that I am an Associate Director of TTC. We are the largest provider of these courses in the country, educating 6-7000 drivers a year. The DVSA,who supervise these courses, say that we are an 'exemplar organisation' ..... in other words if any other organisation wants to see how the courses should be run properly, they should look to our example.