Dividing an estate, testator gifts house to one beneficiary, land around to others

[charlie.b]

The testator gifted 80% of the land around their house, including access up private driveway, to their grandchildren and the house to their daughter. The standalone valuations are house £650k and land £60k. However when you combine the two the house and land valuation is £1.3million. The land also has a large nissan hut on the OS map that could be re-developed into another house. Plus the land and house combined will he much easier to sell. The testator was aware of the much increased value when the two where combined. The executor is saying that the grandchildren are only entitled to a share of the £60k land valuation. Please could someone advise how a professional executor would workout how to divide the £1.3million between the testator daughter and grandchildren in such an estate?

[atticus]

Has a professional valuer given these figures?

If the standalone figures are £650K and £60K, total £10 K, then applying those proportions to £1.3m produces £1,190,000 and £110K respectively. Whether that is an appropriate method, I do not know.