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 14.3% employers contribution has 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 - so, quids in! Regardless, you can of course keep 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.