I think you need to make sure a crystal clear line is drawn between what you are expecting Capita to support directly and what will be supported by your own schools IT team (if you have one).
For example you may dump the LEA an pay directly for capita support, but capita support probably won't be too interested in the day to day running of your SIMS system, such as applying patches and updates to your server and or workstations, they won't create/delete/set up users and permissions and they won't deal with backing up your sims databases or SIMS server.
This is all stuff that the LEA would probably be involved with at some level - you could find that using capita directly could end up increasing workload on your IT staff significantly, that is assuming you have IT staff on site in the first place??
It's not so clear cut. I would not want to lose my LEA SIMS team support. They do hours and hours of SIMS work that I simply don't have to worry about....
It's also useful to find out where the nearest Sims training location is to your school. LA staff may be prepared to make a couple of hundred mile round trip to access training from Capita to cascade on to their schools, but what about your admin/teaching staff? To get Capita to come out and do on site training you are lookin at around £1200 per day.