[Casey76]

Due to a surplus of beans there are some on the plants which are a good 8ins long and have developed beans in them.

Can I still cook them to eat as normal beans (almost treat as runners) or should i go the whole hog, let the beans develop and save them for next year?

[noshed]

Cook a few and see what they're like. You may have to discard the pods if they are a bit tough. Then save some for next year.

[zazen999]

Either.....I leave 95% of mine on to have as shelled beans during the winter but if they are a tad stringy you can use a potato peeler to take the edges off. Just cook for a bit longer.

[bonfire]

Depends how many you have and if you want to carry on cutting them as green beans.
If you leave them to develop you can eat them green "Haricots vert" or dry them and have them as haricot beans.
If you are leaving one to develop it will slow up production of new green pods. So if you've got plenty you can do both and if you haven't you will probably have to choose.