You are right about the split of assets. You mention a separate piece of land, how do you intend dealing with this? Your mother is entitled to 2/3 of the value of the estate, she can appropriate the family home in satisfaction of her share if the other piece of land makes up 1/3 of the estate. This could be sold to pay out the 1/15 share due to each child. The children can also renounce their share in favour of the mother, the child who is not willing to renounce could be paid off from the extra piece of land.
As you see there are a lot of choices, you would do well to consult a solicitor for advice and to do the land transfers if necessary. Stamp duty may arise depending on how the assets are dealt with and their value, a solicitor will be able to help you minimise this liability. You've done the expensive job in getting the grant of administration, the land transfers should be reasonably inexpensive.