I have a new house which I built a year ago and my ensuite toilet has recently started to smell really bad. The sewer smell seems to be coming from the toilet tank and I have tried running water in the jacuzzi and then flushing the toilet but nothing seems to work. I have called my plumber and he seems to think that the toilet needs to be replaced but I have the same toilets in all the bathrooms and there is nothing wrong in any of the other ones. The tank also has a brown film on the inside whereas the other tanks are all clean.

You should remove the toilet and check the wax ring for leaks. A faulty wax ring can leak resulting in waste odors. If the wax ring is OK, then check the flange for cracks. The wax ring and flange work together to provide a "seal" around the bottom of the toilet tank. At the very least all you have to do is buy a new flange/wax ring and reinstall.

If not the above, then if you have a crawl space or basement, check the pipe underneath the toilet if you can and inspect for leaks.

The brown ring in your tank is due to hard water/minerals in your area.