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Personally I'd stay away from the weedkiller. It will end up in your food, your groundwater and your well if you're on one. Pulling them up and mulching would be my preference.
Depends on what you want to do with the site. For lawn, veg and anything ornamental you will need topsoil. The best way of cleaning the site is with a glyphosphate based weed killer. The weeds need to be exposed for min.14days as this is how the herbicide works. The area will need a quick clean off and or breaking up the soil thats there before you apply the new topsoil. Otherwise you may get drainage problems and or soil pans. I would then put it to grass so you can keep it cut and clean until you have the funds or the time to approach it. Best of luck. Din
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