Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
There are varying time limits as you can see here. The minimum is four years from the date that the last IHT payment was made. The articles here and here may be of interest.
HMRC are within their rights to dispute a property valuation and usually they get the District Valuer involved. If I were you I would seek out prodessional advice if you intend to argue against HMRC or the District Valuer. However, taking a case to a Land Tribunal can be expensive in terms of professional fees and court costs.
I hope this helps but let me know if you have any further questions.
The website page behind the third link I gave you appears to have gone down. It's the first result in the google search here. It may be available later.