I'd like to spray part of my lawn to creat some borders which i'm going to rotavate, plant and mulch with wood chip. Can i still kill grass at this time of year and what would be the best product to use?
I have used glyphosate in winter, it does work but it takes a very long time indeed, months. You'd do better to dig now and let the winter do it's stuff on the soil.
I'm going to dig a large new border in our front lawn this winter - I plan to mow the lawn then lift the grass as slabs of turf and stack them upside down in a corner of the back garden to break down and use with compost later. It will then be simple (ish) to dig the beds over and incorporate farmyard manure and compost etc to prepare for planting.
Such a waste to kill it, i would do as Dove says, stack the cut turf and wait for it to compost down, or if you feel energetic you can dig deeper, turn the turf upside down and cover with soil.
I have done this many times over the years and it works as long as you plant the inverted turf at least 1 spade deep to stop it regrowing through to the surface. When you have placed the turf upside down at the bottom of the hole, chop it into smallish pieces with the spade so not to consolidate the upper layer.