Hi Peter,
I just popped into the workshop and a 1/2" ring spanner fits on mine, a sloppy fit but good enough to undo the bolt without marking the corners.
Like Mike says, you need a strange collection of tools for these cars, some metric, some UNF/UNC and probably Romulan or Vulcan for those Planet Clair bolts. It's typical Lotus but in their defense the 70s was a period when the UK was adopting the metric system so they had old imperial stock to use up alongside the new metric from Europe to mix in.
The seat runners are a PITA, especially if rusted so you have my sympathy. I find a 1/4" drive socket can get into tight places which is sometimes useful, and my last resort is to buy some really cheap spanners and grind the nearest one to a "hammered on fit". (I also grind away the outer sides to give a greater angle of movement as well - I is an ingineer
)
Sacrificial spanners, like sacrificial screwdrivers, have a place in my workshop
Brian