My Boat Is Freezing!

I recently purchased a new 473 and keep it moored in the med. I have had loads of problems with the boat but one thing i haven't sorted to date is that it is really unbelievably cold on board. With an outside evening temp of around say 12deg C and with a 3kw fan heater going non stop the boat will just not get any warmer inside. Has anyone got any advice or had any experience in attempting to fit some type of internal insulation.