Strange VVT overheating problem!!

Just returned from holiday towing my small Freedom Microlite caravan.
I've towed it many times before with no problems.
Anyway sometimes the temp gauge starts rising reasonably quickly with no apparent effects on the engine.
To stop this I just open the heater up full for a few minutes when it resumes it's normal just under half way mark.
I'm not losing any water,no steam appears either.Once it triggers and starts rising it doesn't go down again unless I turn on the heater.
It seems to be a few hours of motorway work before it starts if it has been left overnight.If I sit at around 50 it seems ok but sometimes overtaking or if the engine is being worked hard eg long incline seems to sometimes trigger it?
My thoughts are temp sensor/ thermostat sticking or possibly water pump not moving enough water due to worn vanes??
Is the cooling fan viscous and can this have any effect when the car is actually moving??Any one come across something similar??
All the usual signs of overheating are not there eg head gasket,hose etc.
I believe the thermostat is difficult to replace on the VVT??

I had similar on an escort a few years back, nothing obvious like yours, turned out to be the cat starting to break down, but only figured that out after a new one was fitted

You say moving quickly- does it get into the red or just read higher than usual? Mine runs warmer when I'm towing but never gets into the red.

It normally runs just under the half mark even towing.
I went all round the very North coast of Scotland last year with the same caravan and the needle didn't move.
I've not let it get to the red but it gets over halfway and keeps going even if the throttle is eased or I've got over the incline etc.
It only comes down by switching on the heater and once running for about 10 mins I can switch the heater off and the gauge stays just under halfway.
If I don't push it and stay about 50 mph on the motorway it's fine but if I overtake about 60 mph or up a long incline it rapidly climbs again.
I wondered if it could be an air lock but why does it not do it all the time after I switch off the heater?
Another possibility is a faulty temp sensor??
Could the thermostat be sticking and not opening fully?
I've not investigated it yet as I've literally just come back from Scotland today but I understand the VVT thermostat is under the manifold which has to be removed??
It's the fact the water level is not going down that's the puzzle??
Surely if it was a head gasket the temp would go up and stay up and I would lose water??
Does anyone know if the water pump is prone to failure or the sensor??
Remember it's a VVT !!

Mine sits at halfway in normal driving and around 2/3rds when it's working hard. Personally I'd let the needle climb a bit more before doing anything drastic. It may settle slightly higher than normal- as long as it's not in the red it's not actually overheating.

If there is an actual problem my first thought would be an airlock, but I'm not too familiar with the Jimny cooling system and can't find anything online to indicate it being a known issue.

radiator could be blocked, thermostat could be sticking shut, its not electrical if the temp goes down when the heaters are put on. also check the fan is turning on. if the thermostat or radiator where blocked the fan would turn on quicker than normal due to the rise of temperature in the engine or the lack of flow through the radiator.if the rad where blocked it wouldn't be affected on idle, only when the rpm is increased it would start to rise the temp as the water pump is pushing the coolant faster through the rad.

the check if the rads blocked- the top and bottom rad hoses will be hot. the centre of the radiator core will be cool/slightly warm.

sticking thermostat- remove it and run the engine without it, also road test. or take it out and drop it into boiling water. you will visually see the thermostat open and then re close when water cools down.

Radiator fan failure- is it the fan itself working?wiring? put a direct 12v and earth from the battery to the fan wiring connector. it should power up if working correctly. if it does there will be a power feed problem if fan is not turning on when the thermostat opens and stationary.

if the head gasket were blown or on its way out the engine would overheat regardless if towing or not. it would be using water and not leaking onto the ground as it would be burning coolant in the combustion chambers. the engine would puffing white/grey clouds out the exhaust. it would be misfiring in the later stages of a head gasket failure as it is flooding the spark. check the spark plugs to see if there wet. there could also be a mix of oil and water on the dipstick and in the overflow tank(creamy caramel look) from were the gasket has split.