NI contributions on shares

I received some "Restricted Share Units" from (RSU's) my company this month. The RSU's vest in 5 equal tranches over 5 years provided that I'm still with the company. If I leave the company then I lose any unvested allocation.
Tax is effectively paid from the shares themselves; so if I'm allocated a *total* of 1000 shares I only receive 600 (my marginal tax rate is 40%) - so no problems there. However, NI is paid out of my actual salary based on the total value of the shares at the time they were allocated - this means that the effect of this incentive is to signiciantly reduce this month's take home pay. Is this the way these things are normaly dealt with?
Of course, if I stay with the company and the company's share price doesn't bomb, then I'll benefit from the value of the vested shares. But what if I leave the company or the value of the vested shares is much less than the value on which the NI calculation was based - can I reclaim the NI contribution?
Thanks for any advice.
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