Stripped nut on exhaust flange

[rowan194]

Any ideas on how to get this thing off? Both a driver and ring spanner slip. (Haven't tried a normal spanner - that would be worse right?)

I've sprayed some penetrating lube on it and will try again tomorrow when the exhaust is stone cold (it was still lukewarm) but I'm worried the damage is already done.

Guess I need something that can grab the flat parts, rather than the corners...

Funny thing is that it was hard to do up properly (tight) when I replaced my welch plugs a few weeks ago, because it's in quite a fiddly location, so what's made it seize up since?

Thanks for any tips.