I managed to fix it. Well my friend did. Both seats had airbags, so that wasn't the problem. He ended up snapping off the seat position sensors from my old seats and reconnecting them to the new seats, and made up all the wires. He also did something with the pretensions and used my old seat belt sockets. All lights off and all working as they should. Still no idea why my old seats had cut wires hanging out of the plugs though!
I definitely owe him a pint!