Boxster won't start after battery change
 

I changed the battery on my 2003 boxster because one cell of the OEM battery was dead.

When I disconnected the Negative terminal the alarm started sounding. I finished the swap but after that the car won't crank. All lights come on, the radio and aircon work but it won't crank the engine.Is it the immobiliser? How do I reset it?

Many thanks.
Muru

manual or tip?