Fitting a mk3 rear beam

so fitted up my replacement rear axle . and i cant get one bolt in , but the strange thing was on the car i removed the axle from it has 3 X 15mm bolts on each side , this one had 3x15mm on one side 2 on the other and a 16mm bolt too ,. so I'm thinking maybe someone crossed it and put a bigger bolt in ? , i had a look uo the hole and it looks like it has a captve nut and the nut has come un-alighned ? or is it a threaded body ect . just finding it a real pain and when i wound an original 15mm bolt in it goes so fat then jumps the threads , and the 16mm i cant get back in ,

would love to get it back in as it has 3bolts for a reason not 2 .

any ideas other than running a tap up it ? ,

thanks .

mk3 1994 1.0L 4speed coupe