OMG I had no idea there were designated stores for Nectar cards!!!!! I've moved from one store to another this year, with having moved house, and when I spent mine last week there was no problem! Shell that is awful!!!!!
I don't blame you one bit for walking out - I would have done the same!
I hope your repair problems get sorted before next week ((((((((((((((((((x)))))))))))))))))))))))))
Just a thought - Sainsbury's have a Facebook page - I know as I 'liked' it and have their statuses on my Newsfeed. Try 'liking' then post about what happened - you might get something out of it, or at least an apology -plus it shows them up!!!