I'd try a box spanner first as well and assume its going into a rivnut fastening below. If you can get the box spanner on, undo it very carefully and see if it does have a captive fastening if so you are probably ok to go! warning though, rivnuts are often not installed tight enough and after having been used and pulled up often loosen themselves which can make for some frustration. Being an EX land rover man I have a rivnut gun, If you are going to work on a vehicle that has many of these fastenings It would be a good idea to get one if you already don't have one. If you are ever in doubt about the patency of a fastening you can always tighten them up or replace them if needed. When you've removed the seat bolts replace them with button head torx bolts. I go for stainless, yes, I know everyone will tell you they should be high tensile 8.8 bolts but come on 6 stainless bolts would rip the floor out before before they failed in the event of an accident.
