Hi there, i know they can be tempremental about a hair cut, - remeber reading somewhere that you should not cut them back during autumn... i usually do mine during early spring just before they put on new growth, and i just take off the dead flower heads, just above the first knode, being careful not to damage the new shoots which usually appear either side of the knode...

The flower heads can stay on because they are decorative, I also think the flower heads protect them from frost? so in the spring.

yes i think your right Drc, so maybe best to do it once the serious frost is out the way, but buds are still small enough not to be in the way when cutting....

yep, they're right - leave the dead flowerheads on the plant, ignore it till new growth starts to show in the spring and then prune off the flowerheads, any dead bits of stem, and take out any dead wood.