Could you recommend a good residual weed-killer please?

I would reccomend, Pathclear 'season Long' it's a residual weed killer that gives you 3 months protection. you can get it from most DIY outlets.

hi
by rotovating the whole area you have chopped up some weeds roots and spread them even further and disturbed other weeds seeds and also spread them over the whole plot. Rather than use a weedkiller you would be better off covering the areas that you intend cultivating at a later date with black plastic old carpets cardboard etc. I know it doesnt look "nice" but it will prevent light from reaching the weeds and they will die. You can the concentrate on a small patch at a time.
Good Luck sounds like you have a lot of future work to do

If you are planning to use this for vegetables or fruit, definitely do not use residual weed killers. Some veg. are very sensitive to traces of these and you may have problems for years. There's also the question of whether the residues will enter into the plants you intend to eat.
A variation on Dawnrosie's approach would be to remove all the perennial weeds with taproots (dandelion, docks, bindweed etc.) and cover the whole ground with a deep mulch of grass cuttings, or straw, or any other organic material which will rot down eventually. This will prevent the growth of most annual weeds. When you want to use the ground, you can simply draw back the mulch, and plant or sow. You then pull the mulch back around the plants when they are larger to keep suppressing weed growth.
See http://www.motherearthnews.com/Organic-Gardening/2004-02-01/Ruth-Stouts-System.aspx
I use this a lot and it saves a great deal of work, and also keeps the ground moist.

Definitely don't use anything like Pathclear - the name itself tells you what it should be used for. Follow the advise of Dawnrose and Bertiefox and all should be well.