I have two batteries for the M5. After a shoot today, one of them is refusing to charge. It was about half full when I took it off the Movi. Now it is connected to the charger and is alternating between the 1S red light on the charging scale and the main Charge Status red light. Is it a dead battery? I've only had it for a few months.
Alasdair do you have a LiPo checker so that you can tell the actual cell voltages? While it may have had a half charge today was it ever run down to where it was damaged then the damage was done and may have now just manifested itself.
Before you discard it stop at an RC hobby shop or Amazon and get a LiPo checker. Very inexpensive and gives good information on battery state. Example, there are 4 cells in a 4S battery. If you see the individual voltages of 4.0, 3.9, 3.5 and 4.0 then that battery is likely not going to do very well since there is such an imbalance on the voltages.
I have been able to recover a battery that has been unable to charge. It's risky and I am unsure if there are true negative effects but I have been able to do it for anyone interested.