Electric socket tester shows fault

Just back from a lovely weekend near Sheffield, mainly using our caravan as a dormitory while visiting family there.

The Electric Socket Tester plug, (usual sort of orange variety with three lights on), showed "Neutral Fault" .but on first checking with a plug-in low powered table lamp, the plug-in 700w heater, and working up to the (also low-power),kettle, and water heater too, everything worked fine.

We let the site owner know, he was going to get an electrician to look at the post - it's had some problems before apparently.
I am seriously no electrician. but wonder what issues could arise from this Neutral Fault, given that nothing seemed to malfunction.

We were especially grateful for the heater working - it wasn't a hot weekend, and our caravan is not one of your modern highly insulated ones!

Thanks for helpful comments.