Why does my van not start sometimes?

Hi there,

I have recently bought a 1996 diesel 1.9 t4. We went for our first overnight camp yesterday and discovered a few problems! She started fine on the driveway, and we drove her about an hour to the campsite. When we went to turn it back on after checking in, there was nothing! Wasn't turning over at all. We then realised we left the radio on, turned that off and it started fine. This morning, having had it plugged into electricity all night, she started up fine. We then let her run for a few
mins and turned off. When we went back to start it up. Nothing again. We turn the leisure battery on, and she starts str8 away. Then drove about 5 mins, left her again for an hour, and she wouldn't start until the leisure battery had been on for around 5 mins.
Any ideas? We are tearing our hair out and quite upset we spent all this money and can't use it without worrying!


Thanks

Chloe