A Living Will is the old name for an Advance Decision. If you made a Living Will before October 2007 (when the Mental Capacity Act came into force and made Advance Decisions legally binding) then it may not meet the criteria that an Advance Decision needs to meet.
If your Living Will isn’t valid then it won’t be legally binding. This means that a healthcare professional won’t have to follow it. If your Living Will isn’t legally binding then you should make a new Advance Decision.