MBP 2011 Detects Battery but won't charge or run from it

I've got a Late 2011 17" i7 MBP which i've acquired as it was marked as dead and destined for the bin. I dont know how long it's been sat 'dead', but i'd guess at at least 4 years or so.

After a bit of googling and looking through the forums, i determined it was the DC-IN board that had failed. The replacement was P/N 820-6525-A (I think it was) which isn't the exact same P/N as the one that I took out, but apparently they are compatible, and it was more readily available.

Anyway, fitted the new board, and the MBP fired up and all seemed well.

Only issue i could notice was it was charging slowly - but put this down to the fact the battery had probably been empty for a few years.

So, left it most of the day yesterday charging and it got to about 70%. I switched off the charger overnight and while at work today as didnt want to leave it unattended.

For info, the battery shows only 89 cycles from new, with a 95% of new capacity and said good health.

Came to it today, and turned the charger on and left it for an hour or so. Came back to it and it showed fully charged (green light on MagSafe) - which seems quicker than it was charging yesterday.

Unplugged charger, turned on the MBP. It got to about 80% of the loading bar and then died and wouldn't turn on again. Have now plugged the charger back in and it boots okay, and shows battery 81% charged, but says "replace now" and "battery is not charging".

Have tried resetting SMC, disconnecting and reconnecting battery, etc, but battery now appears to be dead. If i press the battery test button (with MagSafe plugged in), all 5 lights flash. If i press it with the battery disconnected (and MagSafe plugged in), again all 5 lights flash. If I press it with magsafe unplugged so its just on battery, there is nothing whatsoever.

Is it that i was just unlucky and the first bit of stress on the battery for probably about 4 years has killed it, despite seeming okay, or is something else possibly giving issues?

Any thoughts, suggestions, etc very much appreciated.

Sorry for long waffle - just wanted to give as many details as possible. Many thanks.