Hi,
If your bonds win then National Savings will either post you a cheque or send you a re-investment bond. They can only do this if they have your current address.
More info here, enter your holders number in the box and see if you've won
http://www.nsandi.com/
Your chances of winning depend on how many bonds you have. Currently the maximum munt is �30,000.
A chance calculator is available here
http://www.moneysavingexpert.com/savings/premi um-bonds-calculator/