Berlingo Forte keeps running battery flat!

I bought a new batery for my Berlingo HDI diesel in December.
On Saturday, the battery was absolutely flat - dspite having a couple of 100 mile+ runs in the last fortnight.
Got a new battery yesterday (Sunday) - left the engine running for 15 minutes to top up the charge. This morning - new battery is absolutly flat!

No electricals have been left on (lights radio etc) - so I'm stumped.

Any suggestions?

Hi Guys
got the AA out. Faulty alternator!

Glad your sorted, well at least you know whats wrong