CG66020 | Losses: loans to traders: payments under guarantee: claims
From HM Revenue & Customs · Capital Gains Manual
A claim for relief under TCGA92/S253 (4) must be made by the guarantor; in practice, any clear indication by the guarantor or his agent that relief is sought in respect of a specific payment should be accepted as a valid notice of claim. On receipt of the claim you will need to check whether the conditions at CG65991 - CG65993 are met by reference to the circumstances prevailing at the time the guarantee payment was made. If they are,
an allowable loss will accrue to the guarantor who made the payment
at the date of payment
in an amount equal to the payment made LESS any contributions payable to the claimant by co-guarantors, see CG66030 - CG66032.