Roundup will do the trick for you Dnnaa.....with no residual left to worry you . If other plants are near either cover them - or use a mask for your spray so that it only goes where you want it.
It is a contact killer.

Daz.

Glyphosate is the way to go. 150ml to 10 litres of water is the recommended rate for couch. Another way to protect young plants is to put an upturned pot or suitable sized container over the plant while you spray. Supermarket bags also work well.

Doug.

Many Thanks. I will embark on a weed pogrom this week end

Masking as suggested works well and also a safe way is to brush on the glyphosate directly to the offending plants.
It is also important to remember that it should be used on actively growing leaves for best effect
John

Brands versus generics. When I shop for any kind of chemical I first find out the generic or chemical name of the thing I'm after then look at all the brands which contain this chemical to see which is the cheapest.
As an example, Panadol is a brand name and Paracetamol is the generic name so look for things with Paracetamol at the strength required.
Another example, perhaps closer to home is Glyphosate. Roundup, Zero etc all contain Glyphosate as the active ingredient. There are quite a few brands around almost all of which are cheaper than Roundup.
Shop around and don't be fooled by marketing hype!

It is always best to cover the soil with some layers of paper or something similar before the mulch goes down. I always put down thick layers of newspaper or cardboard under the mulch. It tends to control most weeds including Couch. Because I worry about water not penetrating before the paper decomposes, I make the paper thinner near the planting zone and sometimes leave gaps, knowing I will have to deal with some weeds working their way through the mulch. I also use Glysophate.

Depending on how deep the root system of your couch is, glyphosate may not be enough to kill the roots off entirely. Runners may brown off and you'll think it's dead but either a month or two later, or next season, it comes back from deep rhizomes This has been my experience where the weather is warm and the area gets plenty of sunshine.

Fusilade worked a treat for me. Fusilade II would be even better if you could get it.

Here is a link that might be useful: Fusilade herbicide