Temp on my JTD was hardly ever above 70 degrees on my 8 mile journey to work - these last (cold) weeks it was struggling to hit 60. Changed my thermostat last weekend and despite scraping the ice off the windows this morning, the temp hit 90!
If you are going to get a replacement, check the number of pins on your thermostat temp sensor - took several attempts to get the right thermostat last week as several motor factors has 2 parts listed for the 1.9 engine same casting and pipes, different temp sensor, but some had 2 pins - I needed 4. None of the suppliers could offer a simple answer when asked for pin numbers. I needed the right part as I could not say if the thermostat or sensor was the problem (only had the car 6 months).
Fortunately I read a thread on here about someone else who had this problem with differing sensors which fore-armed me to the problem.
However, have a warmer toastier car now!