We spoke about this in another thread - new allotment holder's queries;
My plots were covered in couch grass when I took over. I adopted many methods in different areas to get rid of it. Here they are:
Dig up all the grass and stack it up (upside down) in a really high heap for at least 3 years. The plant in the clean soil.
Dig up as much as you can and sow turnip seed really thickly, then rotavate in. Did this twice. Some still came up but only in small patches.
Cover with high quality woven plastic membrane for 2 years (some still came up the year I removed it, but weaker)
Rotavate once a month from March through to October, two years running. Surprisingly, this did not seem to make the problem worse, (as I have read here that it does) it came up but again only in weakened patches. Earthworm population appeared to recover too.
Apply a glyphosate based weed-killer such as Roundup (I used the Ultra 300 version) in Mid May, then scraped off the dead grass after about 3 to 4 weeks and dug and planted veg. This was the most effective and quickest method.
Once the soil was clear I made sure that something, anything, was growing in it. I started sowing poached egg plant, (Limanthes dougasii I think) and find that this is good to keep the ground relatively clear of the twitch.
Hope this helps.
