L322 won't start

Have a 2003 4.4 Vogue. When key put in ignition, key will not turn past position 1 - just has 3 electric attempts and then is locked. I am aware that it can be a battery problem, but has just started and after initially fixing itself upon attaching a set of leads, now will not start for love nor money, despite the battey appearing to be good - any other ideas as justr come out of nowhere. Is it just a new battery needed or is there a common fault.

Disconnect both the Battery positive lead and Earth lead from the battery. Touch both the leads together for 10-20 seconds. This discharges all the retained voltage in all the ECU’s. It’s like powering your desktop computer off and resetting it.
Leave the Leads disconnected. Now check the Battery voltage it’s got to be 12.4 volts or above. If it is then reconnect the terminals if it’s not then charge or replace. Try it again and let me know

Seems to have done the trick - see how we're doing in a week or so