Private composer packages - no valid composer.json was found

I'm trying to load a library I have hosted on BitBucket using composer as explained both in the official documentation and here, but keep receiving the following error:

[Composer\Repository\InvalidRepositoryException]
No valid composer.json was found in any branch or tag of [repository URL], could not load a package from it.

Here is my project composer.json:

{
    "name": "Project name",
    "require": {
        "my-vendor/my-package": "dev-master"
    },
    "repositories": [
        {
            "type": "vcs",
            "url": [repository URL]
        }
    ]
}

And here is the composer.json in my remote repository (that apparently can't be found):

{
    "name": "my-vendor/my-package",
    "version": "0.3",
    "autoload": {
        "psr-0": {
            "NS_": "src"
        }
    }
}

I should mention that both composer.json files are in the root directory as they should be.

Some other things to note:

I've also tried the "non-composer package" approach, whereby I specify the package information in my project composer.json, and omit the composer.json from my remote repository, as outlined in the documentation. This successfully clones the master branch but then results in the following error:

[RuntimeException]
Failed to execute git checkout "master" && git reset --hard "master"

fatal: Not a git repository (or any of the parent directories): .git

However, the package is downloaded to /vendor as expected, so I'm not sure why it's trying to checkout master again.

This is not the way I wish to solve this problem (as I'd rather make use of a composer.json in the remote repository), but it might help identify an issue elsewhere.

Thanks for any help.

EDIT

I've managed to get it working by referencing a package.json over HTTP:

"repositories": [
    {
        "type": "composer",
        "url": "http://localhost/packages.json"
    }
]

The packages.json looks like:

{
    "packages": {
        "vendor/my-package": {
            "dev-master": {
                "name": "vendor/my-package",
                "version": "dev-master",
                "source": {
                    "url": [repository URL],
                    "type": "git",
                    "reference": "master"
                }
            }
        }
    }
}

Is this the only way to get this working? It seems a bit overkill to host my own packages.json file if I'm only going to be using one or two in-house packages.

Regardless, this is giving me the same Git error as I mentioned previously.

EDIT 2

Forcing an error (invalid SSH passphrase) gives this:

[RuntimeException]
Failed to execute git clone "[repository URL]" "C:\workspace\DFv3\vendor\vendor/my-package" && cd /D "C:\workspace\DFv3\vendor\vendor/my-package" && git remote add composer "[repository URL]" && git fetch composer

So I can clearly see what it's doing here. However, it seems after this command runs it cds into the .git directory and tries running:

git checkout "master" && git reset --hard "master"

Presumably to get rid of the composer instance it pulled. However, it's running this in the wrong directory and I can't figure out why..