Last night saw us camping in The Girl's tent in a field in Kent.
Knowing that her tent a Mountain Co-op Tarn 3 has a lightweight somewhat porous nylon/PU groundsheet, we put a full on tarp beneath it.
It rained in the night, and our siting of the tarp wasn't ideal, such that it protruded from beneath the edge of the tent. Rain got on top of it , ran across it and end up in the tent. With hilarious results.
I guess the options are (a) re-proof the existing integral groundsheet and/or (b) cut the tarp to size so nowt sticks out.
Any thing else?