When watching programs on the iPlayer on my Humax the fast forward control is a bit limited. The >> button appears to have no effect, and the >>| button only fast forwards in 5 minute increments. Does anyone know a way to do a fast forward such as x2, x4 etc?
You're out of luck. As you've discovered, +/-5mins is all you've got.
By gaining access to the innards by installing the custom firmware, you can ignore the Humax while it buffers the iPlayer feed at network speed, then save it off as an mp4 and play it later - with much greater control over playback.