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If using an external keyboard will allow you to enter spaces then it's possible it's the keyboard that's broken.
 
If I use an external keyboard I can use the space bar fine.

What would be the best way to fix the problem then? Buy a new keyboard?

I believe the problem is the little silicon piece under the spacebar, its the one that completes the contact between the key and the sensor when pressed, before buying a new keyboard, just give it a try. take out the space key with a thin knife by inserting it in one of the corners and then pulling up. if done gently, it shouldnt be a problem. once thats done, put the little silicon thing back on the sensor and try pressing it with your finger(the silicon piece), if it works then thats great, otherwise i guess you'll have to replace the keyboard sensor.
 
If I'm not wrong, that link you posted for that item is a part of the top case.

It's better if you get the whole top case. Such as this. Easier to install.
 
Thanks for the info, I'm definitely going to take a look. However, why does it need to be a Core 2 Duo MacBook? Mine is an original Core Duo model. Would that make a difference?

Sorry - I guessed it was a Core 2 model - the instructions are just the same.

For a Core Duo model you can user either the Core Duo topcase (orange wire) or the later Core 2 Duo model (Mark/Version 2 - silver wire)

Buy a spudger - it will make life much easier....
 
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