Hi Essie
Your £20,000 will be disregarded for as long as you keep your UC claim open as per guidance below.
6.4 In respect of UC, this instrument provides that any benefit arrears or payment made to compensate for arrears due to the non-payment of benefit, of £5,000 or more which are received whilst a claimant is getting UC, or were received during an earlier award of a legacy income-related benefit and the claimant migrated to UC within one month of their earlier award ending, and would have been disregarded, or were disregarded, in respect of the earlier award (as a result of official error or an error on a point of law), will benefit from the extended disregard until their current UC award ends.
www.legislation.gov.uk/uksi/2018/932/pdf...siem_20180932_en.pdf David