This morning I turned the key - got a click - light went out - turned key - another click - and nothing

Tried it again - second click - short delay of about half a second then it turned over.
It did the same again this evening.
Got back home (after about 25 miles). Stopped the van and tried to start it again after about half an hour.
turned the key - got a click - light went out - turned key - another click - and nothing

Repeated and nothing again and again and again. Will bump start though!
Any idea's? Battery was 12.5V last time I looked but am going to check it again!
Cheers
Russ