From my own experience in NI, you can challenge/contest a will if you are the legal next of kin, and in that instance it would be up to a judge to decide if you should have been left anything.
In addition, it depends on the ages of the great grandchildren, if they are under 18 then their parents would be the legal guardians and they would control the estate until the children were of legal age.
Also, you can't take control of an estate until after probate. Inheritance tax here has to be paid before you can recieve the items/property, and you can't sell the property to pay the inheritance tax. Inhertiance tax is charged at 40% over the threshold of £325,000.
But, like I said, the main issue would be the grandchildren contesting the will, and in the case of an elderly person where it could be suggested that they were not in sound mind, the grandchildren could win. If they were dependant on the deceased financially, for instance, if they lived in her property, then they stand a greater chance of winning, even though grandchildren are not normally eligable to contest a will.
These links should help - they are mostly about Northern Ireland.
http://www.hmrc.gov.uk/inheritancetax/intro/probate-process.htm
http://bfime.custhelp.com/app/answe...s,-probate-&-inheritance---challenging-a-will