I am not a lawyer, but I think you need t osit down with your solcitor and discuss the matter - not over phone.
from what i know, the estate of your grandparents, if they died intestate, goes to all their children in equal proportions. If any of these children are dead, then their share is divided amongst their children.
your situation is more complicated if your paretns paid off the mortgage. You whill need proof of this, but assuming you can get proof, then I think that they have a greater share than the others.
as I said it is very complicated and your solicitor may even need to consult barristers to get more definite advices.
the important thing is not to delay. delays do not help - go in first get whatever advice you can and if you need to do further research or get more documents/information for your legal people, then get it as soon as you can.
delays costs even more money than law!