Hi Sam,
I bought an Alpkit Zhota a few years back which had a crap groundsheet. I solved the problem by getting the dimensions of the tent and cutting an appropriately sized bit of roofing felt for it (slightly smaller than the dimensions, obviously). This is the lightweight stuff, a couple of mm thick. I pitch the tent on top of it and its brilliant, completely keeps the water out and packs up easily with the rest of the tent. Definitely a cheaper solution than buying a new groundsheet or footprint I suspect.