Truely efficient weed killing (ideas please?)

Whatever you do there will be digging involved, a rotovator tends to chop things and in some instances that just means these regrow.

If you do not want a herbicide then steim the tops and dig first, remove everything you can within reason, then rotovate to cut up the ground up and chop anything you did not get. You could well still need to dig again, but I wopuld suggest that for this you do a section at a time.

If you do not mind a herbicide then Glyphosphate in one form or another, maybe stronger then standard for the thistles as these are more "woody" and there is a glyphosphate one for tough weeds which I believe from reading the contents etc is simply stronger when sprayed on. Then let whatever there is die, get rid of the dead top stuff and then dig. You could get one of these flame wand thingy's and burn the dead stems.

You could probably get rid of the main part then section the area up and prepare a section at a time for planting and growing. At least you should then get something from it this year.

How you do the weed removal is up to you.

If you can get some rough dug in between freezings, then the frosts might help break up he lumps a little bit. Maybe ...

  concrete over and grow in pots? 
No seriously, I have no idea, nothing seems to stop them for any length of time.
Grendel

Thistle jungles!, My friend uses WD40 on his Thistles. I might try some on some mares-tail this year, I have nothing to loose 

John points out somewhere on here that ammonium sulphamate is a very good compost accelerator. It is a natural product and breaks down into nitrogen after several weeks, but it kills any plant life. It used to be used to deal with marestail. It would be terrible if you tripped on the way to the compost heap and it spilled all over the pesky marestail...