In reply to Phil1919:
I enquired about this with my insurers for last year's Etape.
They said that my normal travel insurance (Voyager) was fine for general cycling around but wouldn't cover "races" (I know it's more a sportive, but explain that to an insurer), so I took out extra insurance just for the day of the Etape to be on the safe side. They have a little-known bolt-on option for this sort of thing and it cost about £12, I think.
If you're not doing any races, you should be fine.