well while I decide what to do with the old base I decided to drill some hole in the plat iv got and make a start at re-assembly - got the plate bolted down, but both drills ran out of juice so couldn't mount the actual column today

i did manage to repair the damaged nut though
started with this

first i set the base up in the chuck, and clocked off the internal bore

faced off the damaged section till it was nice and flat then did the same to the nut section
i then found a handy lump of ally from my 'might be handy' bin, think it was an old shaft adaptor for an actuator or something
chucked up on a quickly made mandrel and skimmed the outside down to match the bore, added a few small grooves to aid glueing together

left the internal bore as found, thinking the thicker the better

is now clamped up waiting for the jb weld to dry. i might drill and tap a few spots for extra strength but will decide on that later
as for the broken base here it is in situ

current thinking is to jb weld it together, along with some internal plates and then build some sort of box/stilts directly under the column to take the majority of the weight and brace directly to the plate underneath.
would weigh less then concrete and should be easier to construct.