DD frontline hammock (can be used as tent/bivi) and DD tarp is what i've used... just aint hung the hammock yet 

I have a budget of around £100 I will be looking to wild camp on canoe trips one or two nights most probably summer months to start with, as I have an old sleeping bag do I just buy a small tent or do I get a tarp and bivvy bag I could go for both a cheap tent and DDtarp and look to get a bivvy bag later.
Weight isn't a huge issue as it'll be carried in the canoe.
I am new to the bivvy bag idea am I right it is just a weather proof envelope for your sleeping bag?.
Ideas and advice welcome.
Dave