For annual weeds, 30 ml of white vinegar per liter of water, and spray the leaves. For noxious perennial weeds, you will need to use the 4-week stuff, unfortunately!

It depends a lot on what weeds need killing. Glyphosate based weedkiller (most 'off the shelf for the amateur' types) are ideal for the lesser weeds, you know, soft stuff, leafy stuff, but brambles, bindweed etc. although susceptible to weedol and pathclear, will usually need a second application, and there isn't time this year for that to be effective. A dry sunny spell, when plants are thirsty, and ideally this end of the year when nutrients are being stored in a plant's stem and roots, is the perfect time to kill 'em. The weed killer is taken right into the plant's system.
Phil J

But you still need to let the weeds die down and then remove the 'rubbish' to get a clean bed for planting. I think that you would be better to just get on and dig them out and cultivate the ground properly.

I agree with you BB nothing like digging them out. Also there has been some concern in the press recently about a well known weed killer staying in the soil for much longer than stated?

As Bulba says, no matter what herbicide you use (although, realistically, Glyphosate-based products are the only ones that remains on the market in the UK) you must still wait for it to take effect and then clear the dead and dying plant material, which is the 4 weeks that you mention. It's the sort of thing that needs planning a season ahead ideally.

What you'll also find is that by killing off all the growing weeds you'll have created ideal conditions for the germination of countless of their seeds that will have been lying dormant waiting for a chance & space to grow. Using a herbicide over an entire area tends to be just the start of the clearance process, rather than the end.

If you really can't wait then elbow-grease is the only viable alternative and will produce better, more permanent results in the long run.

True. the only really quick, safe, and effective way to get rid of weeds is to dig them out. Corn gluten meal can be applied once the surface is clear to stop seeds from germinating, but only after you are done digging and planting--any digging or cultivating will break the "barrier"--and it won't do anything to stop perennial roots from coming up.