Try a bit of heat if you have access. A dremel might also produce the heat required.

I went to a local garage, who welded another nut onto it to remove it. Subsequently replaced it with stainless bolt with a dab of copper grease.

 

My previous S had the same problem with seized clamp bolts on the exhaust tips.

Had to drill through the centre of the bolt to remove the tip, then re-tap the hole with 10mm thread.

Use high temp grease when re-fitting with a new bolt. (Stainless Steel preferred)

Very fiddly job to get right but can be achieved with patience.

 

Maybe try cutting a slot in the head of the bolt and attack with a hefty screwdriver or flathead bit on the end of a ratchet? But to be honest mine are exactly the same and I think the bolt head will shear before it ever comes loose. The bolts are toast now anyway so don't waste too much time trying to get them out in one piece.

 

Both bolts are sheered on mine, they are completely stuck in situ in the captive nut. I just used a screwdriver to pry it open a little and they pull off. Close it up again on re-fitting and they're quite snug.

I used the britemax twins on mine, the finishing polish is really good.

 

Cut the bolts off, drill the remaining peice of stud out with a 4,5mm drill , re tap to M6, fit with new stianless steel bolts and a blob of waterproof grease.

 

My bolts where the same so I just polished them on the car

 

Don't touch it after a long drive! :-)

 

Well I've just spent two hours this afternoon using Autosol and they look superb!
Thanks for the advice