Stand Alone Leisure Battery Charger

miniminx

I have a Fiat Ducato motorhome, with a leisure battery located under the drivers seat.  I live in a flat and am unable to charge the leisure battery using the orange cable.  I use my van frequently during the winter so am able to gain some charge when driving.
 

However, with the long winter nights the output I'm using is greater than the input so I'm looking for advice on either a power pack which I can attach to the leisure battery to back it up or a portable battery charger which I can charge from my flat then
carry down to my van and recharge my leisure battery.