I thought it was my chargers at fault, but then the iPad charges fine.
I took my phone to an independent shop to have the glass screen replaced (had cracked and smashed during the week - still worked perfectly though).
Switched it off completely on handing it over. When I collected, it was switched on and had somehow been accessed (so password had been bypassed). I have no problems with this - just mentioning it in case it's relevant.
Charge was dropping soon after, it charged maybe a few percent, but then dropped gradually until nothing. It's flat and won't accept any charge. Any ideas?