Power issues with Samsung NP370R5E

So I've been asked to fix a sumsung NP370R5E laptop, which has issues with charging.

The laptop doesn't appear to fully recognise charging properly, in that you plug it in and it shows the charging symbol for a second (pre windows boot up) then reverts back to showing just the battery %.

If the charger is left in then it appears to charge the battery, very slowly, although no charge symbol.

It boots into windows, but keeps flicking between charging and battery and appears to run on battery rather than mains power.

I've taken the battery out (full disassembly) and tried to boot up with charging cable. It fires bios up but then switches off, so I was thinking that this was the charging port or charger. So I replace the charging port. No joy.

I've metered the charger and it's outputting 19v constantly ok. So I'm thinking this could now be a motherboard charging issue, but don't know what to look for next.

Weirdly, when booted into windows (charger plugged in) it shows as intermittently on battery and charge, however, when unplugging the charger it shows as charging!

Can anyone recommend next steps please?