A simple idea that's not very professional, but will work if you don't want to solder.
Normally you have these plugs on leads lying round the house. They're used on mains leads for TVs Computers, chargers etc. You can buy them dead cheap anyway - the ones with a normal 3 pin plug on one end and a black kettle-type connector on the other end. You need a voltmete/multimeter that you can get from a cheapo tool/auto shop for about a fiver. Also available at car boot sales for about a quid (used)
Cut off the kettle connector with about 200-300mm wire left on it.
Strip back the insulation to separate out the two or three inner wires and then strip back the insulation on them until you have about 1cm of exposed copper wire. Hopefully, the inner wires will be brown and blue to correspond with the outer two holes in the plug. the third wire will go to the middle, but you don't use that one.
Cut off the connector from the charger and separate and strip back the wires the same as the other lead.
Join the two pairs of leads by twisting the wires together and apply a small amount of insulating tape to one so that the two wires can't touch.
Plug in the charger to the mains and check which of the two outer holes is the positive. When the voltmeter shows approximately 28v, the red probe is in the positive. If it shows -28v, it's in the negative. Do the same to your battery so that you know which way round is the socket.
If the socket and plug matches, fully insulate the wires with tape, otherwiseswap them over.
Sounds a lot but basically: cut off the two connectors, join the two wires and check they're the right way round. Soldering is better, but twisting them is OK as long as you make everything secure by wrapping with tape.