Yes, signing was indeed delayed, to 19th May. For once the dates are defined in the "definitions" appendix to text:
“Expiry Date” means the later of:
(a) 01:59 on 28 May 2023; or
(b) any such later date that is specified by the Secretary of State pursuant to clause 2.3 (Amendment of Start Date/Expiry Date); or
(c) any such later date to which the National Rail Contract is continued in accordance with clause 2.2 (Additional Reporting Periods);
That refers to and from this bit of the main contract:
2. DURATION OF THE NATIONAL RAIL CONTRACT
2.1 This Contract shall expire on the Expiry Date or pursuant to Chapter 9.4 (Remedies and Dispute Resolution).
2.2 Additional Reporting Periods
(a) If the Secretary of State gives notice to the Operator by no later than 5 March 2023, the National Rail Contract shall continue after such date on the terms set out in the National Rail Contract for not less than one (1) and not more than twenty six (26) Reporting Periods following the date set out in limb (a) of the definition of Expiry Date, as the Secretary of State may stipulate.
(b) NOT USED.
Ooh, that's called a limb, is it?