diff --git a/lib/iris/src/iris/cluster/platform/base.py b/lib/iris/src/iris/cluster/platform/base.py
index ccb94d09f5..03264d7c3d 100644
--- a/lib/iris/src/iris/cluster/platform/base.py
+++ b/lib/iris/src/iris/cluster/platform/base.py
@@ -30,9 +30,11 @@
 
 import datetime
 import logging
+import os
 import socket
 import threading
 import uuid
+from pathlib import Path
 from collections.abc import Callable
 from contextlib import AbstractContextManager
 from dataclasses import dataclass, field
@@ -95,9 +97,17 @@ def find_free_port(start: int = -1) -> int:
             s.bind(("", 0))
             return s.getsockname()[1]
     for port in range(start, start + 1000):
+        lock = Path(f"/tmp/iris/port_{port}")
+        try:
+            os.kill(int(lock.read_text()), 0)
+            continue  # port locked by a live process
+        except (FileNotFoundError, ValueError, ProcessLookupError, PermissionError):
+            pass
         with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
             try:
                 s.bind(("", port))
+                lock.parent.mkdir(parents=True, exist_ok=True)
+                lock.write_text(str(os.getpid()))
                 return port
             except OSError:
                 continue
diff --git a/lib/iris/src/iris/cluster/platform/coreweave.py b/lib/iris/src/iris/cluster/platform/coreweave.py
index 3bd442b37b..0f81cc6b4b 100644
--- a/lib/iris/src/iris/cluster/platform/coreweave.py
+++ b/lib/iris/src/iris/cluster/platform/coreweave.py
@@ -1500,7 +1500,7 @@ def _coreweave_tunnel(
     exits (e.g. konnectivity timeout), it is relaunched automatically.
     """
     if local_port is None:
-        local_port = find_free_port()
+        local_port = find_free_port(start=10000)
 
     proc: subprocess.Popen | None = None
 
diff --git a/lib/iris/src/iris/cluster/platform/gcp.py b/lib/iris/src/iris/cluster/platform/gcp.py
index 082d1a1caa..904b95c19b 100644
--- a/lib/iris/src/iris/cluster/platform/gcp.py
+++ b/lib/iris/src/iris/cluster/platform/gcp.py
@@ -1609,7 +1609,7 @@ def _gcp_tunnel(
     Picks a free port automatically if none is specified.
     """
     if local_port is None:
-        local_port = find_free_port()
+        local_port = find_free_port(start=10000)
 
     labels = Labels(label_prefix)
     label_filter = f"labels.{labels.iris_controller}=true AND status=RUNNING"