Gains: disposal of found objects: not treasure: ownership not resolved
If the ownership of the object cannot be resolved between the finder and the landowner, see CG77520.
CG77546 explains that sometimes each of them will agree to abandon their claim to ownership of the object in exchange for a share of the disposal proceeds. When this happens their share will be a capital sum derived from an asset, being their rights under the agreement.
The capital sum received by each person is chargeable under TCGA92/S22 on the date of receipt. No sum can be deducted as the cost of the right because TCGA92/S17 (2) applies. The right arises from the agreement to share the disposal proceeds of the object, but there is no corresponding disposal of that right.
If, very exceptionally, the agreement was made on or before 31 March 1982, so that the right under the agreement was held on that date, rebasing applies so that a deduction may be available for the market value of the right at 31 March 1982, see CG16700+.