I would spot treat the worst weeds then strim off or flail then have a look at the actual levels, bit of a guessing game until its all mowed down and you can see precisely what state its in.
Ive always treated knotweed using roundup tank mixed with mixture B, whatever you use it will be a long battle. As for nettles, thistles brambles and hogweed, Blaster or timbrel would be my choice.
Hi, Re the knotweed, I have tended to use glyphosate based weedkillers, primarily as its broken down in the soil. However it is a listed invasive weed and there is advice on eradication on environment agency website http://www.environment-agency.gov.uk/homeandleisure/wildlife/31364..... However as Pro Gard says there is no short term fix to its control. Re the meadow I would spot treat the weeds but only where a judicious re-seeding and a meadow cutting regime has been unable to swing the competitive edge towards meadow grassland.