I had exactly the same happen on my old estate once. That time I have four young passengers waiting for a ride home from Majorettes. It was a cold winter night.
I found there was just enough room to lift the seat base a little so that it would tilt a little forward so that i could squeeze a 17mm socket in on a long extension and unbolt the lower mounting bolt.
This is a symptom of another problem with the belt. It is caused by the slot in the top of the seat (where the belt runs through to the inertia reel) being loose or elongated and allowing the top button in the webbing to get past it and down into the mechanism. The top button is designed not to pass through the slot to prevent exactly this problem occurring. You may find the plastic slot moulding is not firmly clipped into the top of the seat and when unclipped the slot can spread, thus allowing the button to pass through.
Hope you are successful at getting the lower bolt undone

If you can't get at it, you may have to try unbolting the seat back hinges on both sides to allow a bit of "give" to help reach the bolt.