HP Elite 8440P Charging Light Flashes 8 Times After Power On, Doesn't Boot

[dave_n_s]

With my Hospice Thrift Store donated computer refurb hat on, this week's queue included a pristine looking Elitebook 8440p and the correct charger, I believe, MM545, which checks good at 20V unloaded between inside and outside of the sleeve.

Plugging in the charger with battery installed does not activate the charging light. 

Pressing the power button for a few seconds and releasing starts the charging light flashing, 8 times.

Then nothing. No sign of HDD spinning (there is one, a 320GB Seagate) or fan running or anything.

Alleged method of pressing power on for a while with no power then reapplying power didn't improve things.

What is this indicating please and in this state can I get to the BIOS or persuade it to boot from a bootable image USB?

Thank you!

[dave_n_s]

Update.....

I eft it plugged in for a couple of hours and noted the charging light was flashing continuously. Pressing the power button didn't have any effect.

I unplugged the power supply and pressing the power button didn't start the power button flashing again.

I plugged the power supply back in and pressed the power button and was back to 8 charging light flashes.