Temperature gauge issues

[Gsparky]

Hi I got a mechanic to change my Engine temperature Thermostat when I had the clutch changed as it had stopped working. Now when I start the engine the red light flashes for a few minutes while the gage goes to the far right then slowly drops back to the far left and doesn’t move again for the entire journey!
it used to work fine for at least 3 years sitting just before the half way point on the gauge.
any ideas on how to fix this, is it a poor quality thermostat that is the problem?

[Mocki]

As you haven’t told us which engine or the age of the van , I think we must assume it’s late WBX , and the problem maybe the wiring has been muddles up on the temp senders in the thermostat housing. 

give more info we may be able to be more helpful