So all the bulbs on the circuit are working yet it flashes fast, maybe its the flasher unit, or did you use those ultra low current LED bulbs? Sometimes they cause problems, infact on cars with bulb failure warnings these will alert you to a bulb out when in actual fact it is fine, as its drawing such a small current it thinks its no longer there...
Could be something along this line, not sure how easy it is to get to the flasher in the Disco having never done it before, but if its similar - once you find it - to the Defender then all you need to do is pull it out and throw in a replacment.