If this is this is the bike you have http://www.bikes2udirect.com/B1111.html?specs=1, which is as you have described it, would look at the cost of replacing the whole bike, vs the cost of the forks, as what you have is barely above BSO level, and replacing the forks will not be cost effective vs buying a complete bike. A basic set of Suntour forks will cost you around £65 + fitting if you can't do this yourself.

For the forks that you currently have, they either work, or don't, if looking to attempt to get them repaired, the cost will be similar, if not more than new forks as suggested above.