Packing up in the rain is one of camping's little pleasures isn't it. I've had to do it many many times but fortunately my mind's blanked out most of the memories. Wait till you get to the next stage where you head off all day in the rain and have to use everything - still soaking wet- for another night of heavy rain. Especially when you find your sleeping bag has somehow got soaked during the day.

Fixing a leaking groundsheet depends on what material it's made of and why it's leaking. Some leaks come from punctures - sharp stones etc can do it easily and you can usually find some material to patch it with. I have an old Vango Force 10 tent (about 40 yrs old now) that I still use regularly with a groundsheet made out of something like pvc. That's been punctured loads of times and has just been patched with some bits of material I got from Vango. It's still watertight.

I had another tent where the groundsheet was made of proofed nylon - a sort of heavier weight version of the flysheet. That was fine until you used it in really heavy rain where the ground was saturated and then the whole of the groundsheet seemed porous - no single leak point, water just seemed to soak through it. I never found anything that worked to fix it and just used a cut down pvc tarp (a few quid from boot fairs or Machine Mart) as an under layer.

The "footprint" approach is a good idea anyway if you're putting the tent up on anything other than a putting green as it's sacrificial protection for an expensive tent. I've usually cut mine to be about an inch or so smaller all round than the tent groundsheet - lay it on the ground, put the tent up on top and go round with a stanley knife. Ideally it would be a bit bigger so you could lift up the edges to form a kind of tub but in practice I've found that some bit usually drops down and forms a kind of funnel to channel water in.