Welcome to the forum Sue. And glad you like the video.
Words are a problem! what is fresh to one person is old to another and I hope I have not confused too many people by my video comment. I put down less rotted manure but it was still a year old, more straw visible than usual.
Some gardeners do spread fresh (say less than a month old) manure and it has a weed suppressing effect but I think that quite a few nutrients may be leached out of it. The composting process makes them less soluble in water.
Starting was you are in midsummer, I don’t think that couch grass will be sufficiently killed within two months to remove a tarpaulin. I would leave most of it in place until early spring, just try a corner for your plug plants in March.
Check in winter to see if any couch shoots are still growing, if not all is well, otherwise leave tarp and plant e.g. potatoes, squash through holes perhaps.
