If you do any work for building contractors or subbies, they have to verify you with HMRC and it is their responsibility to deduct the tax from your labour element, so show your labour separately from materials on any invoice. They must then pay this amount over to the HMRC on a monthly basis, and they should give you a periodic statement detailing how much they have deducted and paid over. You will need this statement to offsett against what you may have to pay over when you submit your tax return. It is important you get this statement in case they don't pay it over and something happens to them as i think you may still be liable.
To verify you, they will need your unique tax reference Nr (UTR nr) which HMRC should give you.
This is how it should work although i hear of people doing it wrong.
If you just work direct for businesses or householders then, as others have said, setting aside 20% should be more than enough.
I'm an MD for a small main contractor so we do this all the time and these are the rules.
Al