So the news is that it is a "clutch bearing" that is worn. The strip down means it needs a new clutch kit, total job cost £1046.
As it is a bearing, my warranty (typically) doesn't cover it. However, as it's been such a pain in the neck to get this far, Renault UK has offered to pay 85%, leaving me with a £160 bill and a 12 month guarantee on a new clutch.
I've decided to accept the offer to get the whole saga done with. Seems like a very good offer from them, but I'm still unsure why the garage didn't find a dodgy bearing 6 months ago when they put the new gearbox in??
Anywhoo, looking forward to getting the car (might even be today) and having a proper drive!!