Then you are still a part-time employee. If that's the case I think you can say you no longer wish to drive and return to working in the building at your inside rate. I believe you would have to wait two years before being able to drive again but could be wrong.
So you never go back inside the building, just drive? If so are they adding a full-time position each year because of the number of days you drive as a TCD? I thinks it's 200 days driving to make them add an employee and 156 for you to get full-time vacation and holiday pay.