Britfan - I concur with those above who have used good quality SS fasteners, and when buying, look for the grade. When I inherited my Commando, it was littered with stainless bolts. Some of these had no grade stampings, and some of these indeed stretched and self-welded. I replaced all such fasteners with good quality SS fasteners. Be careful at shows and autojumbles, where there are boxes of shiny SS fasteners on display - all that glitters is not 'gold'.
I have used good quality SS fasteners for decades on a range of classic machinery, without any problems. Even before I learnt that one should use copper grease on the threads, I never had any of these fasteners self-weld or stretch permanently. I will buy from engineers suppliers, or if online, I look for the grade. One might be cautious about using SS for wheel spindles (especially on Commandos where the spindle fastens to a stub axle) and other highly-stressed parts but I have never heard of any failures due to a spindle being stainless. If I was confident of the grade and the alloy and if the threads were rolled, I would use SS for spindles.