Batteries naturally discharge over a period of time.

MA Batteries should be stored at <65% charge.

At what level should the batteries be recharged back to max (and then down to 65% if still in storage), when flat, probably not, 25% left, 10%, or when the 1st LED starts flashing (0-13%),or Is lit (13%-25%), what is the perceived wisdom here?

If you read the support page from DJI, you will see the problem is fully answered there.
10. How do I update the firmware of the Mavic Air's Intelligent Flight Battery?

11. How often should the Mavic Air's Intelligent Flight Battery be charged and discharged?
The intelligent chip of the battery is regulating its volume at all times. It does not need to be fully charged or discharged manually on a regular basis.
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12. Do I need to turn off the Mavic Air's Intelligent Flight Battery before charging it?
13. What should I note when using and storing the battery?
To store the battery for a long period, discharge and charge the battery every three months.

https://www.dji.com/support/product/mavic-air

Good to know thanks

Battery should store in around 40-50% if not in use for long time. I use my battery to 20-30% and don’t recharge till next flight. As I almost fly every week.

the storage level for lithium based batteries are 40%, and the DJI smart batteries will drop down to 40% in self discharge after 10 days without usage/pressing the battery indicator button. also lithium batteries discharge very very slowly on their own, their very good at keeping the exact level of charge for a prolonged period of time, over 6 months, wich is the maximum amount of time a lithium battery can safely(without damage) sit in storage

Thanks all, so to summarise

For an MA battery
    If battery level > 65%, after 10 days battery will self discharge until <65%

    Once at 65% the battery will slowly self discharge and should be recharged  before that battery shuts down (Hibernation) at 10%, or every 6 months whichever is sooner.
    A Battery with <10% charge can be determined because none of the LEDs light up when the button is pressed. (From MA Manual)

    Always fully recharge the battery as this ensures that balance charging takes place and therefore prolongs the life of the cells.