Has anyone got any suggestions on what the problem could be? Could my amateur wiring be to blame?! As far as I know if the network is badly wired it just won't get any connection rather than slowing it down. I know some of the faceplates are a bit loose from DIY work that's happened around them.
Take the two machines and join them directly with a shop brought patch lead and try to copy a file. This will rule out your wiring.
Once youve done that then you can move onto other issues.