Immobiliser issue?

Strange thing happened today with 2.4 TDCI 110. About to head off on school run, turned key, nothing, no lights nothing. So I prodded batter terminals, turned key and the lights came on, but engine would not turn over - nothing. Thought maybe the fob battery, although I'm sure I replaced it 6 months ago, pressed buttons on fob - nothing, no alarm activation nothing. After about 15 minutes, tried turning key again and heard a very faint effort to start, then we started and off we went. This happened once before, I've replaced the battery since...