I work for Capita and I worked previously in the BSF side of the business for 3 years.Standard On-site's will be kept busy by normal day to day jobs on the desktop side that your already do. you will be probably be put into the mobile engineer role as there are no network admins in BSF (not on site anyway). I mobile engineer doesn't necessarily mean you will be driving every where,it just means that you support other schools in your area. you will probably be given domain admin access once they have assessed your skills. If you do not like your new role there are loads more you can move to in capita. If you are a real techie, have a look at becoming a technical implementation specialist (like me), you get to play with all of the best kit that we use and you get lots of perks with the job. if that doesnt appeal, there are tons of other jobs!
dont worry about capita taking over, its not as bad as you think, only different.