foggyjames wrote: Heat + WD40 helps a lot - Andy isn't wrong there

Try a Metrench style socket or shifter on the bolt/nut (the type of tool that contacts the flats of the hex nut/bolt, not the corners) - they have a bit of movement until they contact fully but you can apply a safe amount of high torque with them without rounding off the nuts. Try tightening the stuck nut/bolt first slightly then undo. The whole exhaust stud may undo like this if it's a LH thread. A completely cold engine will mean no heat expansion binding the threads together. Soaking with WD40 or a finer spray overnight will give the best result. If that doesn't work try shocking the threads by hitting the bugger with a hammer and punch, then use the spanner. The Metrench sets are expensive but worth their weight in gold for jobs like this. I have had them undo some badly mangled bolts that I thought I'd never get undone. You could also try welding a lever to the stuck bolt - often the intense heat from welding will pop the threads apart to loosen it. I'd have to agreed with Mac too - heat and WD40 is always fun and should shock the thing loose. Good luck.