In reply to PaulJepson:
I had an 8 year old Marmot tent. Water used to come straight up through the floor.
I sent it to these guys:
https://www.scottishmountaingear.com/repair-service/tent-repairs
The interaction was a bit strange. First they contacted me to say that they had tested the flysheet, and found that it was leaking. And they don't do flysheet replacement. Therefore they wouldn't replace the groundsheet.
Anyway after a bit of back and forth, I managed to convince them that I was happy with the condition of the flysheet.
They replaced the groundsheet, and having since camped in the wet, I can say I am very happy with the work they did for me.