It is against the law to pay under the minimum wage, here are some notes on enforcement. …
It looks like first port of call is the HMRC, and then the Employment Tribunals, best act as if the 3 months minus a day deadline applies.
- Announcing the 2020 rate
- https://www.acas.org.uk/national-minimum-wage-entitlement/if-an-employer-does-not-pay-minimum-wage
- https://www.acas.org.uk/early-conciliation
The HMRC are the enforcement agency