wifi not working (Virgin customer uk)

I need you guys help, i have a iphone 4, i had sky broadband which worked fine on my ihpone, since i upgraded to virgin super hun 30gb it never works, loads for about 30 secs then thats it, its still connected but wont load. this is a common problem as i have searched loads of sites for help but all i get is to change the wifi channel which ive done, almost used all of them now and still no luck. My misses has the 4s and has the same problems, has anyone had this problem and solved it?