tax on social plan payout

WTF do you mean by "social plan"?

I presume the social plan you are referring to is a redundancy settlement. If so I'm pretty sure it is taxed as income using the income tax rate from the previous tax year. Best to check with your HR department and / or accountant though.

You won't be far out with your 50 percent expectations. That was about what I paid, the only consolation being it won't happen again.