Finally, you realise that you have chanced upon a special breed of shopper. Those for whom the period just after Christmas is a season of many happy returns. Inside those bags are the patterned sweaters, babydoll negligees and skimpy knickers which seemed so appropriate to the giver, yet appalled loved ones on Christmas morning.
M&S has a famously liberal policy of accepting returns. So liberal indeed that many of those who feel a cash gift is insulting, will have popped in just before Christmas, grabbed a few pairs of loud, diamond socks, confident that the money may be discreetly reclaimed at a later date. There is just that little problem of the long queue at the vast M&S refund section. Thank God there is a shop assistant dispensing boiled sweets to keep us good humoured. "Any chance of a gin?" I ask.
Cindy Colfer is here in line at the Oxford Street store, victim of an ill-chosen Christmas jumper. "He got me a size 8," complains Mrs Colfer, 46, from South-east London. So what's the problem? "I'm a 12," she replies through clenched teeth. Well-intentioned, seasonal flattery? But Mrs Colfer is unimpressed: "He just picked it off the rack without looking and thought that would do."
Poor Mr Colfer. He had been trying. He doesn't usually venture out alone to buy clothes for his wife. "He gives us the money and tells us to get something for Mum," pipes up his daughter, Susan. "But he forgot Mum's birthday this year, so he felt he had to go out on his own and make amends."
For his double error, Mr Colfer did get off more lightly than others. "She says they are too long in the leg," sighs Paul, 37, an accountant from south London. He shows me the black satin trousers he bought for his girlfriend. "I got the size right, 14. But I've been told off for not knowing she's a 30 leg. I thought 311/2 would be OK. They looked fine to me. For a bloke it's all right if trousers go over their shoes. But I'm told women like to show their feet."
Paul has left his girlfriend at home in bed on the bank holiday to arrive as soon as M&S opened. Obviously chastised, he has promised to be back in time to bring her coffee in bed. No, he will not give his full name. "It's bad enough doing this without everyone knowing about it."
Maura, 30, and her husband, Chris, 30, are still arguing about the gold satin underwear he gave her. "I was disappointed," says Chris. "She had told me that those were the ones she wanted and then she changed her mind." Maura mutters that she said no such thing.
This underwear problem keeps cropping up. Melissa Braiden, 32, from Eastcote in north-west London, is completing the annual ritual of returning a set of bra and knickers from her husband Sean. "I like what he chose," she says. "It's just that he always buys me underwear. I've still haven't used last year's lot."
But I do not find a single man returning a lover's present. There is the occasional wife bringing back a shirt that is too large. "He just gave me a look of resignation and said it was too big," confesses Linda Judd.
Disgruntled women far outnumber the men. Perhaps it is because women are such good present givers, such great assessors of size. Or maybe we wisely keep quiet.
For some the return trip is just the inevitable and amusing result of poor communications. "Every year my husband's sister gives him extra large vests which don't fit him and she gives me black tights which I don't like," declares Maud Jones, 69. Why don't they explain, I wonder. "Oh we wouldn't want to offend her."
Tanya Nouril, 29, is clutching a thong from her mother. Husband, Michael, 28, is, as usual, returning his ma-in-laws traditional gift of socks. "These days, mum even wraps up the receipt," she says.