--- a
+++ b/tests/acceptance/update_external_studies.py
@@ -0,0 +1,55 @@
+"""
+This is a utility script which it's convenient to have sitting alongside the test it
+supports but which isn't actually executed by the test suite — hence the "no covers"
+"""
+
+import shutil  # pragma: no cover
+import tarfile  # pragma: no cover
+from fnmatch import fnmatch  # pragma: no cover
+from pathlib import Path  # pragma: no cover
+from urllib.request import urlopen  # pragma: no cover
+
+from .test_external_studies import EXTERNAL_STUDIES, STUDY_DIR  # pragma: no cover
+
+
+def update_external_studies():  # pragma: no cover
+    if STUDY_DIR.exists():
+        shutil.rmtree(STUDY_DIR)
+    for name, config in EXTERNAL_STUDIES.items():
+        target_dir = STUDY_DIR / name
+        tarball_url = f"https://github.com/{config['repo']}/tarball/{config['branch']}"
+        download_files(target_dir, tarball_url, config["file_globs"])
+        create_dummy_files(target_dir, config.get("dummy_files", []))
+
+
+def download_files(target_dir, tarball_url, file_globs):  # pragma: no cover
+    for name, read_bytes in get_files_from_remote_tarball(tarball_url):
+        # Strip the arbitrary leading directory from the tar path
+        path = name.partition("/")[2]
+        if any(fnmatch(path, pattern) for pattern in file_globs):
+            out_path = target_dir / path
+            out_path.parent.mkdir(parents=True, exist_ok=True)
+            out_path.write_bytes(read_bytes())
+
+
+def get_files_from_remote_tarball(url):  # pragma: no cover
+    with urlopen(url) as stream:
+        with tarfile.open(fileobj=stream, mode="r:gz") as tarobj:
+            while tar_info := tarobj.next():
+                if tar_info.isfile():
+                    yield tar_info.name, lambda: tarobj.extractfile(tar_info).read()
+
+
+def create_dummy_files(target_dir, dummy_files):  # pragma: no cover
+    """
+    Ensure any necessary filepaths exist (e.g. output files that are referenced in a
+    dataset definition
+    """
+    for dummy_file in dummy_files:
+        dummy_filepath = Path(target_dir) / dummy_file
+        dummy_filepath.parent.mkdir(exist_ok=True, parents=True)
+        dummy_filepath.touch(exist_ok=True)
+
+
+if __name__ == "__main__":
+    update_external_studies()