Vectra 1.9 Diesel difficulty restarting

Hi

Bit of a strange problem. I can start my car without any problems first time, however if i was for example to nip into a shop and come out after 10 mins it wouldnt start. It turns over but continues to do this until the battery dies. If i was to leave it say for 6 hours between journeys it would start no problem. Just seems to be anywhere up to 2-3 hours between that this problem occurs.
Doesnt seem to matter what the weather is and ive had the battery checked and its fine.

Any suggestions

Thanks