Stick 'em now Nick, if the ground's not frozen or waterlogged. Just add a little compost to the hole when you plant them and keep an eye on them for a week or two to see that they're ok . I moved a little clump recently from a front border into a new bit at the back and they're at just the same stage and looking very happy.
I agree, and a bit deeper than they are in that pot.
Would it be a good idea to separate them or keep them together
That depends on how much root there is. If you tip them out, and the roots are all tangled I would leave them...but if there is only a small amount of root you could separate them by washing off the compost in a bowl of water.
I would leave them together. They'll look much better rather than one hear and one there. When I planted mine the first time I got them I planted them out as if I were grwoing for production. Then I realised they looked really loanly and a bit sad.
Yes, keep them intact. Half bury a label with them too. Split when you have developed a decent size clump