Good luck. A couple of years back the garage I used overtightened the locking nuts on my focus and snapped the key off. Took them 2 days to remove. Welding a nut on top will be your best bet. There are specialists that remove these for a living, could be worth having a look online see if there is one local. Or govto a local big tyre place, ats or quick fit etc. They may know someone who can do it. When I fit them I only nip them up, don't need to over tighten at all.