diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 8913a005..8da40770 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -3,3 +3,4 @@
 | GitHub user | Real Name       | Affiliation | Date       |
 | ----------- | --------------- | ----------- | ---------- |
 | yaswant     | Yaswant Pradhan | Met Office  | 2026-07-17 |
+| jfrost-mo   | James Frost     | Met Office  | 2026-09-04 |
diff --git a/github_scripts/get_git_sources.py b/github_scripts/get_git_sources.py
index 313313ac..883fd635 100644
--- a/github_scripts/get_git_sources.py
+++ b/github_scripts/get_git_sources.py
@@ -280,10 +280,10 @@ def get_unmerged(loc: Path) -> list[str]:
     return files.stdout.split()
 
 
-def check_existing(loc: Path) -> None:
+def check_existing(loc: Path) -> bool:
     """
-    If the repository exists and isn't a git repo, exit now as we don't want to
-    overwrite it
+    Return whether the repository already exists. If it does but isn't a git
+    repo, exit now as we don't want to overwrite it.
     """
 
     if loc.exists():
@@ -292,6 +292,8 @@ def check_existing(loc: Path) -> None:
                 f"The destination, '{loc}', already exists but isn't a git directory. "
                 "Exiting so as to not overwrite it."
             )
+        return True
+    return False
 
 
 def clone_repo_mirror(
@@ -309,25 +311,30 @@ def clone_repo_mirror(
     - loc: path to clone the repository to
     """
 
-    if loc.exists():
-        check_existing(loc)
-    # Clone if the repo doesn't exist
+    fetch = determine_mirror_fetch(repo_source, repo_ref) if repo_ref else "HEAD"
+    if check_existing(loc):
+        # If not provided a ref, pull the latest version of the current branch.
+        if not repo_ref:
+            run_command(f"git -C {loc} pull")
+            return
+        # Update existing repository.
+        run_command(f"git -C {loc} fetch origin {fetch}")
+        run_command(f"git -C {loc} checkout FETCH_HEAD")
     else:
-        command = f"git clone {mirror_loc} {loc}"
-        run_command(command)
-
-    # If not provided a ref, pull the latest repository and return
-    if not repo_ref:
-        run_command(f"git -C {loc} pull")
-        return
-
-    fetch = determine_mirror_fetch(repo_source, repo_ref)
-    commands = (
-        f"git -C {loc} fetch origin {fetch}",
-        f"git -C {loc} checkout FETCH_HEAD",
-    )
-    for command in commands:
-        run_command(command)
+        # Clone if the repo doesn't exist. If the mirror is local we don't copy
+        # the objects to make it much faster.
+        try:
+            # Adding `--revision {fetch}` to the clone would be more efficient
+            # due to avoiding an unnecessary checkout, however we need to
+            # support versions of git older than v2.49.
+            run_command(f"git clone --shared {mirror_loc} {loc}")
+            run_command(f"git -C {loc} checkout {fetch}")
+        except SubprocessRunError:
+            logger.error(
+                "Cloning from local mirror failed. "
+                "Check your local guidance on how to set up mirror access."
+            )
+            raise
 
 
 def determine_mirror_fetch(repo_source: str, repo_ref: str) -> str:
@@ -361,7 +368,7 @@ def clone_repo(repo_source: str, repo_ref: str, loc: Path) -> None:
     - loc: path to clone the repository to
     """
 
-    if not loc.exists():
+    if not check_existing(loc):
         # Create a clean clone location
         loc.mkdir(parents=True)
 
@@ -377,7 +384,6 @@ def clone_repo(repo_source: str, repo_ref: str, loc: Path) -> None:
         for command in commands:
             run_command(command)
     else:
-        check_existing(loc)
         commands = (
             f"git -C {loc} fetch origin {repo_ref}",
             f"git -C {loc} checkout FETCH_HEAD",
diff --git a/github_scripts/tests/test_get_git_sources.py b/github_scripts/tests/test_get_git_sources.py
index 081281fd..66112be3 100644
--- a/github_scripts/tests/test_get_git_sources.py
+++ b/github_scripts/tests/test_get_git_sources.py
@@ -147,7 +147,8 @@ def test_check_exists(setup_sources):
     Test check_existing
     """
 
-    assert check_existing(setup_sources / "SimSys_Scripts") is None
+    assert check_existing(setup_sources / "SimSys_Scripts") is True
+    assert check_existing(setup_sources / "does_not_exist") is False
 
     with pytest.raises(FileExistsError):
         check_existing(setup_sources / "empty_dir")