diff --git a/apps/predbat/plan.py b/apps/predbat/plan.py
index 2a1e25a38..0bcd49205 100644
--- a/apps/predbat/plan.py
+++ b/apps/predbat/plan.py
@@ -4113,10 +4113,12 @@ def optimise_swap_export(self, record_charge_windows, record_export_windows, dro
                                 set_window_start(self.export_window_best[window_n_target], self.export_window_best[window_n_target]["end"] - (window_length + window_length_target))
                                 is_combined = True
                             elif export_mode_of(export_limit_target) == EXPORT_MODE_TARGET and window_length_target < orig_length_target:
-                                # Partial combine
+                                # Partial combine - move export time from this window into the target, so
+                                # this window shrinks by what the target gains. Growing it instead pushed
+                                # its start back into the slot before, through any manual override there (#5423)
                                 amount_to_move = min(orig_length_target - window_length_target, window_length)
                                 window_length_target_new = amount_to_move + window_length_target
-                                window_length_new = amount_to_move + window_length
+                                window_length_new = window_length - amount_to_move
                                 self.export_limits_best[window_n] = min(export_limit, export_limit_target, key=export_limit_sort_key)
                                 set_window_start(self.export_window_best[window_n], self.export_window_best[window_n]["end"] - window_length_new)
                                 set_window_start(self.export_window_best[window_n_target], self.export_window_best[window_n_target]["end"] - window_length_target_new)
diff --git a/apps/predbat/tests/test_optimise_swap_export.py b/apps/predbat/tests/test_optimise_swap_export.py
index b419b82a7..af917c0db 100644
--- a/apps/predbat/tests/test_optimise_swap_export.py
+++ b/apps/predbat/tests/test_optimise_swap_export.py
@@ -195,6 +195,29 @@ def run_optimise_swap_export_tests(my_predbat):
         failed = True
     my_predbat.calculate_best_export = True
 
+    # ---------------------------------------------------------------------------------------------
+    # Partial combine regression (#5423): moving export time from an earlier window into a later,
+    # trimmed one must shrink the earlier window, never grow it backwards into the slot before it.
+    # Here the slot before is a manual demand override, so growing into it exports through it.
+    # ---------------------------------------------------------------------------------------------
+    now = my_predbat.minutes_now
+    export_window_best = [
+        {"start": now, "end": now + 30, "average": 50.0},
+        {"start": now + 30, "end": now + 60, "average": 50.0},
+        {"start": now + 80, "end": now + 90, "average": 50.0, "start_orig": now + 60},
+    ]
+    setup_swap_export(my_predbat, export_window_best, export_limits_best=[100.0, 0.0, 0.0], rate_import=30.0, rate_export=50.0)
+    saved_manual_all_times = my_predbat.manual_all_times
+    my_predbat.manual_all_times = {now}
+    try:
+        my_predbat.optimise_swap_export(0, len(export_window_best))
+    finally:
+        my_predbat.manual_all_times = saved_manual_all_times
+    middle = my_predbat.export_window_best[1]
+    if middle["start"] < now + 30:
+        print("ERROR: partial combine grew window 1 back to {} into the manual slot ending {}, windows {}".format(middle["start"], now + 30, my_predbat.export_window_best))
+        failed = True
+
     # ---------------------------------------------------------------------------------------------
     # Ordering regression (#4478): the export swap must run after every pass that can enable an
     # export, otherwise the exports the plan pass / optimise_solar add are never
diff --git a/coverage/cases/random_results.json b/coverage/cases/random_results.json
index c8944e63e..15b0282e0 100644
--- a/coverage/cases/random_results.json
+++ b/coverage/cases/random_results.json
@@ -27,17 +27,17 @@
     {
       "id": 1,
       "seed": 1,
-      "metric": 1164.5356,
-      "cost": 1154.7627,
+      "metric": 1164.4733,
+      "cost": 1154.5992,
       "cost_pv10": 1429.766,
-      "cost_pv90": 1017.5324,
+      "cost_pv90": 1018.3924,
       "import_kwh_battery": 8.2145,
-      "import_kwh_house": 20.3602,
-      "export_kwh": 10.5863,
+      "import_kwh_house": 20.3497,
+      "export_kwh": 10.5909,
       "soc_min": 0.38,
-      "soc_final": 4.762,
-      "battery_cycles": 16.5804,
-      "carbon_g": 20357.15,
+      "soc_final": 4.7532,
+      "battery_cycles": 16.4913,
+      "carbon_g": 20352.73,
       "runtime_s": 1.161,
       "failed": false,
       "error": null,
@@ -65,17 +65,17 @@
     {
       "id": 3,
       "seed": 3,
-      "metric": 628.401,
-      "cost": 646.8817,
-      "cost_pv10": 624.2009,
-      "cost_pv90": 579.9167,
+      "metric": 628.3622,
+      "cost": 649.2458,
+      "cost_pv10": 623.6014,
+      "cost_pv90": 582.2808,
       "import_kwh_battery": 0.0792,
       "import_kwh_house": 0.0,
-      "export_kwh": 8.3425,
+      "export_kwh": 8.1858,
       "soc_min": 0.3907,
-      "soc_final": 1.2819,
-      "battery_cycles": 11.1137,
-      "carbon_g": 9719.92,
+      "soc_final": 1.4502,
+      "battery_cycles": 10.9505,
+      "carbon_g": 9769.74,
       "runtime_s": 0.343,
       "failed": false,
       "error": null,
@@ -179,9 +179,9 @@
     {
       "id": 9,
       "seed": 9,
-      "metric": 477.0656,
+      "metric": 477.3462,
       "cost": 581.2203,
-      "cost_pv10": 805.6605,
+      "cost_pv10": 807.2194,
       "cost_pv90": 254.3911,
       "import_kwh_battery": 16.7377,
       "import_kwh_house": 5.1054,