hi stevie, well you in luck, im a joiner and just joined today so hi all . how bad is the floor? tbh in personal experience if the floor is a little topsy turvy but not hurrendous then just use the "combi-floor" underlay, and in the places where its lower just put a 2nd 3rd or even 4th layer to get it all roughly level then lay your floor. you can also buy 12mm thick underlay, whcih looks like big green sheets, which you could use if the floor is terrible but its cost a bit more