Are you sure it's not the wiring at fault rather than the front indicator?
I have that rear unit on the rear of my bike, but i left the original indicators on as well as they are brighter than the LED's, plus i want people to see me turning anyway, the more the merrier.
I didn't need a resistor but think you need it if the originals are removed?
Not sure about the legalities though, might be worth phoning whoever does your MOT's?