How can I fix a Heater Thermostat to my Laguna?

I have a 1997 model Laguna 1.8RT, I had a new heater matrix fiited about 2 yrs ago , however just recently when I put the temp gauge upto full the air being pumped around is only luke warm, I pressume it is the thermostat thats going or gone.
Is it easy to repair and how much is a thermostat?.