In reply to ablackett:
Not impressed with the response from Terra Nova, I said,
"As far as I can remember the groundsheet has always leaked, however, out of the 40-50 nights i've used it it's only rained a couple of times so i'm really not sure when the problem developed. I used it this weekend and in moderate rain, pitched on well drained land it ended up with puddles in the tent which had come through the ground sheet."
They said,
"I would suggest the best option would be to replace the groundsheet rather than put patches onto holes. The approx. cost for this will be £200"
It looks to me like they haven't even read my email properly, it's leaking through the groundsheet, not through holes in the groundsheet, £200 repair on a £379 tent seems to me to be priced at a level to make it not worth the bother.
https://www.scottishmountaingear.com will replace the sheet with a slightly heavier duty one for £89 so i'm going to go with that. Thanks for the advice.