what size spanner do spark plugs for a 2cv use?

At a quick remembering I think its 21mm, I may however be wrong.

I'm fairly certain I normally use a 22mm deep socket which is a slightly loose fit.

Nope, you're bob-on. It's the usual "large plug" 21mm - same as the sump/gearbox plugs - but most socket sets'll have a "proper" plug socket in them, with the 'ickle rubber widget in to keep hold of the plug as you pull it out.

Remember - fingers only to get it in as tight as you can, then just a quick nip with a ratchet or similar - and only put plugs into COLD heads.