I am posting to hopefully help others / share my experiences / seek any further help.
Yesterday after I plugged a charger in my FP2 and l left it, suddenly I got the beeping to indicate it was on 15%. I had plugged it in at 70% charge a few moments earlier. The charger I was using was a small one and I had to twist the cable slightly to lay the phone flat on the desk. Initially I thought it was some sort of battery problem, as I have had problems with the battery holding charge. But now thinking about it I think it was the twisting that has caused something to come loose inside. I have tried 5-6 different chargers, with success once - but it got to 70% charge and started discharging again, with the lightening bolt flashing indicating (I think!) there was a connection problem.
I have now successfully ordered a battery from a store in another country, with the help from other kind people on here. I think it is a fault with the bottom module so have ordered a new one of these.
Hopefully this sorts it all out!