heater sensor inaccurate

If you are sure it is the gauge, it could be the temp sending unit. They usually have one wire in them and are screwed into the block. Their main purpose is to tell the gauge what to say.

ok - so how do I access the block?

I am referring to the engine block. You will have to Google the location of the sending unit for your year make and model. Understand that there is also a coolant temp sensor that is designed to tell your cooling fans to come on. Be sure to Google the one for the temp gauge. It will be the one with only one wire. The fan one has 2. Sometimes when I have trouble locating a part I go to my local auto parts store and ask them. Lots of times they will be able to tell you the location for your car. When you Google, make sure to include your engine size too. Good luck.