Twitch grass is another name for couch grass - this isn't a problem in lawns anyway, because the grass is cut so often it doesn't cause trouble. Other weed grasses do though - since you're laying new turf, it is necessary to dig over the area once the old turf is removed, so the best way to ensure a new, weedfree lawn is to dig out any and all pernicious weeds - although you needn't, as I said, worry too much about couch grass.

I have to disagree with Bamboo on this one. We have couch coming up in the lawn and it spreads from it into the borders. It is also very visible in the lawn grass as the blades are a different shape, size and colour. Supposedly cutting the grass gets rid of it, but this bit of lawn has always been cut and it is still there.
As said though, digging is as good a way to go as any, Remove all traces of weed roots. Then leave the area fallow for a while and spray with Roundup anything that comes through before you lay the turf.

I'd certainly agree that if there's couch grass near the edges, then it may well migrate into the borders. Otherwise, depends how rigorous you are about clipping the edges and mowing frequently I guess; I never had a problem in my own garden, but certainly, some of my clients over the years who didn't keep up too well with the lawn regime ended up paying me more because of the necessity to remove grass growing into the borders, couch particularly.

I does not matter how rigorous one is about clipping the edges, the stuff goes from lawn to border well below the level of and clipping. Our lawn is cut to the lowest level on the mower rather more often than I enjoy. It is treated with Weed killer and regularly spiked and scarified and the couch grass is still visible and spreading. Indeed I am about to put weedkiller on that section to see if that kills the Couch and then I will resow with lawn grass.
All our grass areas have concrete edgings extending a good six inches vertically. The couch roots spread another six inches below that. How deeply do you go in tending the edges?

2-3 inches generally, Owdboggy - I tend to push the soil out the way and clip with the edging shears, moving it back afterwards if necessary and often taking bits of turf too if they're not quite in shape. But I'm talking about a lawn edge with no edging of any description - much more difficult if it butts up against something, or has a preformed edging material. I'm just wondering why the couch grass in my own lawn was completely unnoticeable - what was noticeable were the other weed grasses, forming those rapid growing clumps in the turf.

Been talking to a friend, they have a 12 inch wide mowing strip made of old Victorian tiles between their grass and borders. Saves a lot of work he says. However Couch grass roots still spread from the meadow area into the borders. You must be very fortunate with yours, wish I had that type!

A neighbour where we used to live left his dug over plot for a year before turfing, taking out every weed as it appeared! But as he lived next to a field it probably wasn't long before dandelions invaded his new lawn anyway.

My lawn is paddock grass. So I have all sorts of things growing in it that would not be tolerated in most lawns. I do have couch grass everywhere. It is a real pain. I edge my lawn regularly, (monthly) with an edging iron. the sides of my lawn are about 5" drop to the beginning of the border. The couch grass roots I have dug up at the edge and in the border have been as deep as 9- 12". Bl@@dy stuff is driving me mad, especially as the worst location is all around the path that is around the house. I wish we could lift the whole lot up, house and path, so I had at least a smidgeon of a chance to get rid of it all. :-) For some i have to dig it out but other areas I use a glysophate (roundup) which does the job in one go, but then I use the agricultural type. It may take a few applications of the domestic form.