As above. I use a jack underneath, but have had success in similar situations by levering against the arm to pull the partially fitted nut upwards to engage the taper more fully. There are tricks with G clamps and even on some cars, valve spring compressors, but on the Mini these will only fit the track rod end, where a similar problem exists. Modern track rod ends may have a socket for an Allen key to prevent rotation while you nip them up with a spanner, but I have yet to see that sensible innovation on the Mini ball joints. Not easy and cheap to manufacture.
But as can be inferred from previous suggestions, you only need to get the taper pushed together moderately tightly (having taken them apart you will know how tight they grip!) and anything suitable that you have to hand will do.
But please check that some moronic parts supplier has not given you metric nuts for a UNF thread. It is not unknown, and it is even more common on the driveshaft/hub nut.
Oh, and as you are likely to have the same problem with the bottom ball joint some time in the future, the trick is similar, just jack under the arm where it meets the tie bar, until it is taking a portion of the car's weight on the taper.
You can use a ratchet strap around top and bottom arms on some cars. Never tried it on a Mini.
Have fun!
Edited by tiger99, 04 January 2017 - 03:10 PM.