I left full time work aged 58, and have been paying VCs quarterly since then. It costs roughly £200 every 3 months. I have a Gateway account, and can see each year how much my pension has grown. They credit the payments in April, and show you two bars - one with the maximum (predicted) pension in the year you'll retire, and the other with what you would get if you retired then on the contributions you have paid up to now. I can see that mine are equalising.
It is annoying, as I have over 40 years of contributions, but because of being opted out I had a shortfall, when a friend of mine who has never worked will get a full pension based on state credits made when her children were young, a few voluntary contributions and the new grandparent credits. It all seems so very unfair, but there is nothing I can do about that except continue to pay.
I do some consultancy work on a regular but ad hoc basis, and because I am still under State Pension age I pay NI on my earnings - I usually get paid in two months of the year. The voluntary contributions don't get reduced accordingly, though. There seems to be a flat rate, regardless of other payments.
It is good to watch the differential between the two bars decreasing though, and paying this way is psychologically easier than shelling out thousands of pounds in one go.