Ford Transit 280, yr 02 electrical problem?

The first time this happened was a month ago, the van was parked up while on a delivery with flashers going & locked. unlocked central locking went to start and starter motor just kept turning engine half then cut out then another half a turn then cut out if this makes sense, noticed that immobliser red light was on when starting, so got out relocked the unlocked started immediately.This second time, same thing happened but this time she would not start, rung garage they suggested battery was flat as this time while trying to turn over central locking was locking & unlocking doors.So to save me call out charge and because i had jump leads they said try a jump start first. However headlights were full on & blower was full on, with no dip in lights or reduction in blower when tried to start. Ok then put leads on got a jump it started first time.took leads of within 10 seconds again it started about 20 times and i put all electrics i could think of including blower, cigar lighter, headlights. cant be battery surely. I have been around never come across this behaviour from a battrey before. Any suggestions will be gladly received, if you have had similar a fault.Thank you.