Thanks for additional input.
I did speak to the store manager on Saturday (I said I didn't want to go home without having exhausted all my options). I spoke to "Kevin" who is head of non-food. He just showed me a page on his phone which showed the terms and conditions on the website and repeated what I'd already been told. Said if I didn't like it I would have to take it up with head office.
LadyDee, agreed. I wouldn't either (not that they would be likely to fit me!) and as mentioned earlier I'm not arguing that this condition is unreasonable in itself (the Tesco staff said if they did accept them back they'd have to go straight in the bin). Re price, agreed it isn't a huge amount (about a £10-£15 for three pairs), so I doubt I'll take it much further other than drafting an email to "head office" and waiting to see if the store do call me back after talking to the original cashier.
My main reason for drafting this thread was to check to see if I was in the right or not after having been fobbed off for 15 minutes by three different members of Tesco staff of varying seniority. I'm not about to go to the small claims court