For Christmas I got an exped downmat (Lite 5M) and its great, but will I need to use a groundsheet with it if its going to be outside my bivi?
Thanks
Ben

For Christmas I got an exped downmat (Lite 5M) and its great, but will I need to use a groundsheet with it if its going to be outside my bivi?

I would ... if the ground's dry and 'clean' then you'll be fine but that's pretty rare, a polycro or similar sheet will keep your mat dry(er) and clean(er). The really important thing is to check the ground well for anything sharp/pointy and remove it.

I was working on a few ideas for a 'new' groundsheet yesterday