Some electronics help...

The bodywork of the car will be earthed (i.e a - ve) connection.

I cable to +ve (i.e fusebox - on the fused side of the fuse), and -ve to the bodywork would work.

All circuits should be fused. Either via a fuse in the fusebox or via an inline fuse. You should *NEVER* wire anything direct to the battery. There is a lot of amperage there in the event of a short circuit and a good way to introduce the fire brigade to your car.

If going via the existing fusebox ideally you would use a free fuse location and dedicated fuse. If not then you could tag onto an existing one providing you do not exceed its capacity with it's current and your new load combined. For your purposes you will need a permanent live feed otherwise your locks will not work with the ignition off. A vehicle circuit diagram, if you have one, would be helpful to you.

As for the black wire... That can go to just about anywhere on the body/chassie as virtualy all cars use the metal body of the car as a common ground. There is usually a common earth point in the area of the steering column but really anywhere should suffice aslong as you can get a good metal contact.

Bottom line though is if you are not confident in what you are doing then best get someone who is and whose competence you trust.

Vin.

the central locking wire kit has a wire of its own (a little black box type thing) reckon this would make it ok to tag it onto another fuse?

If you mean by 'a wire of its own' an inline fuse in the positive (red) wire - then fine. As long as it is a fuse. Can you open the box and see a replaceable fuse? Some 'systems' if they generate an inductive pulse when they operate will have a 'choke' in the wire to stop such pulses being passed back and interfering with the rest of the cars electronics. These can look like 'little black boxes'. They would not be openable though. If in any doubt there is nothing to stop you fitting an inline fuse. It may turn out to be superfluous but it would do no harm. With an inline fuse it would not matter which side of the fuse in the fusebox you picked up as long as the wire you connect to is capable of taking the load (its own plus the extra). In any case an inline fuse is a good idea as you can then connect to a higher rated circuit and your new circuit would still be protected by its own likely lower rated fuse.

Check your fitting kit instructions. They should give some idea if an inline fuse is part of the supplied kit.

Vin.