Donal, you need to fully charge it NOW and then either keep it on an automatic charger which switches to maintainenece mode when fully charged, or recharge it every 2 months over the winter period
If you cannot keep it on charge from mains, consider a solar panel charger. even in these dark days there should be enough sunlight to keep it topped up. Also check that nothing remains connected and thus taking a small current - could be built-in radio perhaps ?