I was looking at this for a client recently. It's £660 or so to get about £150 of pension per annum, which is a good deal.

However, you only need 30 years of contributions to get full state pension and anything after that is wasted makes no difference. You probably have about 20 now, and nearly 30 years to get the remaining 10 or so. If you voluntarily pay now, and then pay employment or self-employment contributions in more years than you need, the voluntary ones would be better off in a private pension.

You can probably ignore voluntary contributions for the next 15 years or so at the very least.