Take advice again, tell your solicitor:
There was an agreement (oral contract?) to replace the car.
The repair they are suggesting has already been attempted and it failed.
The dealer needs to be given a deadline to resolve the complaint.
Get advice on the timescales that govern what actions you can take.
There is a difference between you "rejecting" the car and the dealer or SEAT "replacing" the car. And different criteria and timescales apply.
On Monday you will have owned the car for 9 days and the timescale will be critical for some actions. Get professional advice from a solicitor or trading standards on what options are available to you. Don't take advice from the dealer on what you can and can't do.
If your only option is to let them repair the gearbox you need to find that out for certain now!
with it only being the mechatronic unit that is nackered i cant get a new car because apparently this has happened to over 25000+ vw/seat/skoda/audi vehicles in the past 3 years! so its a common fault for the vag dsg cars :/ so they have said its an easy fix and they are willing to get it sorted on monday so i can have my car by tuesday? the way the manager has explained to me the mechatronics unit they tried in my car to fix was from a different car they had in the garage, and these mechatronic units are built for your car using the chassis number ?? so that one they put in wasnt correct anyway, i have spoke to seat customer support and they have told me the exact same thing that i have got to let seat uk fix my car because its only a small problem and easy fix, and im not liable to get a new car, id only be able get a new car if it was something major like the entire gear box and was going take 3/5 weekws replace, so thats the story :/ looks like ive just got put up with it now and hope tht my car is returned by tuesday and running fine like when i picked it up. i have also been looking on the golf mk5 r32 and audi TT forums and its very common on there and easy to fix.