Can't get the last bolt out to release exhaust?

[Esoteric1]

Can't get the last bolt out to release the exhaust. I am in the process of changing them for stainless steel bolts.

The 12mm ring spanner I used worked fine with the other 3 but slipped on the last one so i used a needle file to file it down to 11mm (This was easy due to corrosion). The 11mm spanner slipped so i thought i would just leave so as not to make it any worse.
Any ideas as to what tool i can use to get the bolt out, cheers!
I will shopping on the flea markets in the next couple of says.

[mid life crisis]

Hi, you can get sockets (and some tims spanners) that are called six wall drive. These put the preasure in the middle of the flat instead of the corners, also good for rounded bolts.

[416cc]

If you have any old Imperial sockets use an impact driver with a socket that's a tight fit.

[Esoteric1]

Cheers I have managed it now.
I pulled up at a mechanics garage and put a set of grips on it and released it.

Replaced with stainless now.

Bit dark!