I agree with 'din' particularly about giving the weedkiller the chance to work on the weeds before covering.
I produced a bit of a write up on people starting an allotment from scratch.
OK what you plan is not quite an 'allotment' but many of the things you will have to do to develop your garden will be similar.
I hope the following link & related links will help you with your development work.
Thanks for the replies. The whole site is now yellow/brown with dead weeds and have been digging up the biggest ad thickest hill grass clumps and large stones. I have hired an 8hp rotivator for the weekend and planning on going over the whole site with that then levelling and compacting it before adding a top layer of quality top soil.
Will let you know how i get on over the weekend