What's involved?
Is it just a case of calling your insurers?
I have heard people mention a green card but not sure what that is
Phone your insurers, the odds are with comprehensive insurance you'll be covered for 180 days.
All insurance sold in the uk automatically covers you for the legal minimum required by any EU country. This means you have third party cover for anywere no need to notify your insurer.
If you want to increase the cover to the same level you have in the uk , probably comprehensive. You will need to ring your insurer and may have to pay an additional cost or they may have a free period as mention above most max at 90 days as you are then considered non uk resident and niether cover would apply. Difficult to enforce that last bit though.
As above you get the minimum cover free, call your insurer to find out if you have fuller cover (free or for a price) or if they will not extend it.
As above always call the insurer to check, you might not be covered fully comp unless you pay a premium. I always ask for a green card too; the French police especially will make your life hard if you don't carry one, even though it is not necessary for pretty much all of Western Europe.