Can you post the actual model number of the appliance please?
Roma 100 covers a few models.
The actual model is RM 10 FRS
It's quite an involved strip down of the appliance.
You have to take the lid off first (if fitted).
Then take the cooktop off.
Take the knobs and buttons off.
Take the facia off.
Take the thermostat off of the facia.
Take the thermostat phial inside the oven off.
Take the rear panel off.
Take the phial out the rear and thread the phial and capillary out of the appliance.
Fit the wiring from the thermostat to the new one.
Thread the capillary and phial back down the appliance and into the oven.
Fit the phial back in place in the oven.
Fit the thermostat to the front chassis.
Refit the facia, the knobs and buttons.
Refit the cooktop and the lid.
It sounds quite easy but there is a lot of parts to be taken off to achieve this.
If it's the dual fuel version and there's an electrical part to the cooktop, be careful to disconnect and make an accurate diagram of the wiring to the electrical part of the cooktop.
If it's all gas, there may only be and earth wire to disconnect.
Good luck with it.
Have a good look at it and see what can do.
If you get stuck, post here and let me know.
Hope this is of some help
Let me know how it goes or if you need more help