You've got 10 weeks from the last date in that month to get your NHS pension payment to your local Area Team. Go beyond that, and they can reject your payment.
But strictly speaking you're not free to spend all that un-cashed money; the employers contribution must to be returned to the practice. That said, if you're using NASGP's T&Cs, there is a clause that allows you to reserve the right to charge an extra 20% to compensate you; in which case you can then keep what would have been your employer's contribution, and use this and what would also have been your personal contribution, and you'll be wise to invest it in a private pension pot - but it'll be unlikely to accrue the same benefits as it would have inside your NHS pension.
Use NASGP's locum Terms & Conditions which includes a clause to receive compensation if their delay causes payment rejection.
If you simply forgot to make the payment, you are free to try submitting it anyway, advisably with a covering letter apologising for the delay and including any unforeseen circumstances.
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