Greenhouse (Aluminium frame) nuts and bolts

The type that sit in the track are specials with the heads formed to locate when rotated, they look a bit like a letter "T" when seen from the side one way.

They'd be cropped nuts   

like the WD40 idea, thought about that as I was struggling to slide a bolt that actually fits...    Did think about filing too, but I also have a motorised grinder that might do them quicker...My Dad has already warned about the appropriate technique to use, so I'll be careful - thanks!

Get a can of lithium grease and give the tracks a quick squirt, (I also did the bolts as I assembled) If you still get the bolts in the channel, put some extra in there. This will save you needing to worry about the cropped nuts and make attaching staging / shelving easier.... Wish I had. 

Forget trying to grid them down - it will either end in tears or a molten mess - use a hand file.

Them nuts is bolts. And to make a few , grip one in a pair of molegrips and cut the shoulder off the bolt head on opposite sides with a junior hacksaw.

Them nuts is bolts

Indeed they are, I wondered why I winced, involuntarily, at the thought of cropped nuts.