Steve, as an added precaution, I had a stone cold bottom hose after a slow motorway run at 60-70mph, and the OBD temperature was reading well within safe limits, about 90 Celsius. Car did have HGF under heavy load around that time to complicate my explanation, but when I tested it was not overheating (early stages of HFG and gentle drive). When the car was overheating the OBD reading would shoot up. It it might be worth someone verifying whether a healthy L series car will have a cold bottom hose on a gentle motorway run, backed up by an OBD reading to ensure the thermostat is opening and keeping the temeprature about 90 Celsius.
I am thinking that as the L series puts so little heat into the coolant under normal gentle running, so even with a working thermostat, it may only need to open a trickle to provide a small amount of cold water from the radiator, so the rad bottom hose will stay cold.
A stuck 'stat will give a cold bottom hose too, but I think due to the low heat generated by the L series under light load, being a DI Diesel, a fully functioning cooling system might need so little coolant flow through the rad on a slow cruise, that it will give a cold bottom hose too.