"I thought that the monies would be put in his client account on date of death pending the grant of probate??"
Try and think about this logically. The money was in your grandmothers account. How do you think it finds its way into the (solicitor's) client account? That only happens when the Grant of Probate issues - only then does the executor have the entitlement to call for the money from the bank.
Unless there has been an inordinate delay in dealing with the estate ( and frankly, unless you are a major beneficiary and / or with sufficient information to know what if any, difficulties were encountered), you really are not in a position to level accusations of delay. It would not be usual to call for interest. Anyway, if you were bequeathed a sum of money, that is all you are entitled to.
My advice to beneficiaries always is if you have a real concern, go and pay for proper legal advice about it. Most solicitors take a pragmatic view.
mf