Does not start

Hi. My Hiace ended the previously day faultlessly but when trying to start the next day it tries to start but slowly shuts off as though the batteries are flat. I put it down to needing new batteries but then noticed a little color disc that indicated that the batteries are fine. I connected some jump leads and without even starting my other vehicle and the Hiace started first time.

I know literally nothing about cars but I really think it is a battery problem but am confused by the indicator saying they are good.

Any pointers would be really appreciated before I go and spend the best part of £200 on batteries I may not need.

Thanks.

Jim.