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I eventually managed to get it to start up by holding down the power button and plugging the magsafe in after 10 seconds. It didn't recognise the battery so I powered down opened it up, disconnected and reconnected the battery. It started up first time but the fans we're working over time so I reset the SMC and all seems ok again. Recognises the battery, charged to 100% and fans off.
It's been my experience that the batteries can get so fully discharged that they destroy themselves. I leave my very elderly Mac laptops plugged in permanently rather than risk it.