well Mags it looks like you've taken on something there but its putting it back to an allotment plot that's what your looking at is it not
well its not impossible its achievable with time and effort quite a lot of us gardeners have experienced clearing up someone's mess when taking on a new allotment plot so what to do well I'm going to tell you what i would do first of clear the plot of all that damn rubbish that's been dumped and those carpets and poly sheeting i hate those things being used on an allotment
then set to cut those brambles down don't forget to wear a good pair of thick gloves and a sharp pair of secateurs then you can clear all the weed rubbish left by others so hopefully you will then have a more clearer picture of the next task
now given the time i would be spraying the hole plot with roundup then after a couple of weeks when you can see the weeds are dying of its time for the spade dig it over remove any roots or dodo in the ground then after all that you can then think what your going to do next year
plan out your plot and you may need to feed the ground could be an idea to get a soil testing kit you could always think about growing some green manure or possibly get some other manure to put some goodness back in the ground
after doing all that you could find the ground will get back to normal growing conditions i had problems on one plot with soil capping and i covered it with a thick layer of horse manure and it perfect now no capping good growing soil
as for your greenhouse well you've got all winter to sort that out and bring it back to life anyway that's my advise i forgot to say if you one of those who don't us chemical sprays then its back to the good old spade and fork good luck so with your allotment