diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 65f2e4058..dd7f44160 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -7,3 +7,4 @@
 | jennyhickson    | Jenny Hickson    | Met Office  | 2025-12-10 |
 | mo-marqh        | Mark Hedley      | Met Office  | 2025-12-11 |
 | MatthewHambley  | Matthew Hambley  | Met Office  | 2025-12-15 |
+| tommbendall     | Thomas Bendall   | Met Office  | 2026-01-15 |
diff --git a/components/science/source/kernel/inter_function_space/sci_wth_to_w0_average_kernel_mod.F90 b/components/science/source/kernel/inter_function_space/sci_wth_to_w0_average_kernel_mod.F90
index 571985cd3..cde614eaa 100644
--- a/components/science/source/kernel/inter_function_space/sci_wth_to_w0_average_kernel_mod.F90
+++ b/components/science/source/kernel/inter_function_space/sci_wth_to_w0_average_kernel_mod.F90
@@ -83,9 +83,11 @@ subroutine wth_to_w0_average_code(nlayers,                 &
     ! Internal variables
     integer(kind=i_def) :: df, k
 
-    do k = 0, nlayers
-      do df = 1,4 ! Loop at the Bottom
-        field_w0(map_w0(df) + k) = field_w0(map_w0(df) + k) + field_wth(map_wtheta(1) + k)*rmultiplicity_w0(map_w0(df) + k)
+    do df = 1, 4 ! Use bottom four W0 DoFs in cell
+      do k = 0, nlayers
+        ! Use rmultiplicity from bottom layer to ensure appropriate average
+        field_w0(map_w0(df) + k) = field_w0(map_w0(df) + k)                    &
+          + field_wth(map_wtheta(1) + k)*rmultiplicity_w0(map_w0(df))
       end do
     end do