When My vauxhall frontera overheats with thermostat in place when i take the thermostat out it runs normal would you say that the thermostat is not opening?

Absolutely. I would definitely replace this.

You can check this by making sure that when you remove the thermostat it is cold and then dropping the thermostat into a bowl of boiling hot water and see if it reacts..... if it doesn't then that's the problem! If it does then flush the coolant system through with some radiator flush and then flush through with clean water before renewing antifreeze and water.