Hi Dot. I'd never heard of 'smart pay pension' but a websearch reveals that it is salary sacrifice, so I can explain that for you.
With salary sacrifice schemes, you agree with your employer not to take part of your pay. The proportion that you do not take gets paid directly into your pension (this is going to be a defined contributions scheme I assume, not a defined benefit (final salary) scheme).
You don't pay either tax or employees NI on the sacrificed part, but your employer doesn't have to pay employer's NI on the value of your sacrificed salary either - so it should be paying the amount of NI that it saves into your pension pot in addition.
It works best for 40% taxpayers.
I don't see how one can end up with a higher take-home, unless the impact of putting the sacrificial pension contributions can be shown to produce the same additional pension as the State Second Pension, for a smaller level of pension contribution (net effect of the salary sacrifice on your take-home) than compared to your take-home when paying the full whack of employee's NI that buys you into the State scheme. Your company needs to provide you with illustrations to show this, but I don't see how it can be guaranteed.