I am also in a similar situation looking for some answers.
As far as I can make out (relevant to your situation) CGT is payable at 33% on the gain between the probate figure of 350k and the eventual sale price of 420k. Obviously your estate agent did you a huge disservice by lowering the probate valuation. The CGT is due and payable by each of the 4 beneficiaries in the current tax year by 15th December 2015, though each has a personal exemption of 1,270. Thus the calculation would be: 420,000 - 350,000 = 70,000 @ 33% = 23,100 /4 = 5,775 - 1270 = 4,505. Each beneficiary's net CGT payable would be 4,,505 in the current tax year by 15/12/2015. I am unsure what other deductions might be allowable (valuation fee, estate agent's commission, etc.).