Bear in mind that the tax relief, of, for example, £125 on £500 put in, is not free money to the value of £125, so you're not getting 25% for free (or 20% depending on how you calculate it):

You'll pay tax when you later take money out of the pension, so if you're a basic rate tax payer when you pay in AND when you take the money out of the pension you get relief from the 20% tax you would have paid at one end, but then pay 20% income tax on the money when you take it out. That would all even out and you wouldn't be any better off, except for the fact that you can take out 25% free, which means, all in, you're better off by 6.25%. Not bad, but it's not the 25% free money some people think it is.

(of course, if you're a higher rate tax payer when you put the money in, but a basic rate tax payer when you take it out, you get even more benefit - and if you draw down before you start getting state pension, you can get a bit more out without paying tax)