Hijet engine surging then overheating. Any suggestions?

I have a 1.3 Hijet van that we use regularly on our nursery.

Yesterday, we noticed the van was running hot to the point that it started overheating. Coolant was low, engine oil was low etc. The van has had a bit of a service this morning so new coolant, new engine oil, belt tightened etc.. When starting up now, the engine will rev higher than normal and then on return to idle, it continuously surges. I have taken the van for a short trip around the yard to see if the issue cleared however no luck and back to overheating.

I have now noticed the fan doesn't cut in as it should when the engine heats up. Can I have some suggestions please on what to try next. I'm assuming the thermostat is the main cause of the problem but I can't see why this is causing the engine to surge so much all of a sudden.

The van has been reliable and run every day for the last 18 months so don't to do too much just yet on what could be a simple or expensive fix.

Thanks for your help!

Chris

I suspect the coolant may have been filled incorrectly. There are several posts on here on how to do this, particularly by HighlyJetted and Raggy. I have some new t/stats in stock (somewhere) if you need one and can't find locally.

How low was the coolant to start though - could/can you hear surging from the header tank? It may be that the head gasket has gone, sorry to say.