That rear cylinder head nut

It is a tight fit. Some barrels are more awkward than others. Use a 12-point ring spanner with the outside ground down if necessary. It's a 5/16" BSF (1/4" Whitworth) nut, same size as the heads for the 4 bolts next to the spark plugs. Favourite is a 1/4"BSF & 5/16" BSF double ended cranked ring spanner which you can use on the nuts at the front too.

Britool spanner I nicked from my father half a century ago. Yes, the handle does touch the cylinder fin when putting it over the nut but it clears when in position.

…got me by then and now. Most came from George Stephenson era!  

One little tip is the first 1/8 of the spanner does not engage with the nut so flush it off ( it’s only there to slip onto a nut when you are out of sync with the flats).  This reduces the issue of interference with fins when positioning it.  It also stands out in your tool collection as a special and should be identified as so.  I have mods to a few bits and pieces , if I don’t label them I forget what they are for.   

Jon

.. from a car boot sale / autojumble and chopped it in half so it didn't collide with carbs etc when torquing down after rebuilding. Even though I no longer have a Commando it's still useful for awkward nuts.