I'm not sure SEAT will be your best bet.
Any small garage isn't going to be well equipped to fix the problems, they usually only have skills to tidy cars up to sell them (new tyres, clean, oil change, mot etc) on and don't fix actual faults.
Additionally they won't want to spend time and money AFTER they've got your cash.
You can either kick up a stink (if you paid by credit/debit card this can be quite easy) with the garage and keep trying to get them to fix something they probably don't understand.
OR
I'd cut my losses, find a GOOD vag specialist and they'll probably sort it inside a day. Cheaper and better than seat, not free like the garage that sold it but, but it seems to me like your car has had enough people avoiding spending money on it.
As the saying goes 'you get what you pay for'
After 10 years driving various cars and experiencing various problems trying many different ways to getting them fixed (dealers, mates, RAC/AA etc etc) I'm very pleased with my Leon and even more pleased having found a garage which understands the car inside and out and actually fix things and know about the cars faults. Not only that they don't charge for investigating faults only fixing them. MAJOR PLUS.
Either way - good luck, once you've got over this hurdle you'll love the car!