I work for M&S over summer and christmas periods (if you manage to finish your training to a 'qualified' level they let you stay on the higher pay bracket if you work there again in the space of a year).
The impression I got of the roleplay scenario was that you'd have to be stupid or lack any confidence to fail it. I doubt they're all the same but my scenario was as follows:
The interviewer first informed me that we would be playing out a scene where she was a customer looking for help. I was then given about 20-30 laminated cards showing the name, image and price of foods/drinks. After 5 or so minutes she returned with a basket containing several more cards. This was where the 'in character' part started. She advised me she was hosting a dinner party and required a suitable selection of foods and drinks, the main caveat being her lack of access to any cooking facilities. Obviously the idea was to get her a varied choice of ready to serve foods. I found this pretty easy - although just about forgot to check the cards she already had - which contained things like uncooked chicken.
Although I imagine they vary the scenarios across the country I would have thought in a big company like M&S they try to standardise as much as possible. If you get something like the above just categorise your items (so you're not left shuffling through everything to find those sausage rolls and crisps you remembered). Don't bother trying to memorise everything.
M&S basically expect you to smile, be helpful and treat your customers like they're gold. It gets tedious after a few months, but given the attitude of some stores on the highstreet I can understand why they hammer it into you so hard.
Anyhows, it's a nice place to work and the discount is uber. M&S food ***.
EDIT: Oh, and I've done the ASDA 'Daz' one too. It wasn't anything like that.
