Key figures, turnover, expansion rates, company history, key pivotal moments in the history, competetors, brand knowledge/awareness, company ethos, annual report info detailing future plans, swot analysis prior to going in there as you will do that for example. Know how the stores work and how they keep costs down, and how there seasonal middle isle crap has a massive fast turnover of cheap chinese tat that people love and impulse buy. These are all areas that being knowledgeable in will help out.
Carry yourself confidently, dont mutter, speak clear and concisely, have some authority in your mannerism, look corporate but not over the top, maybes slip in a non conventional pair of shoes, belt, shirt etc soo you look and feel confident in what you wear and not like you are a burton menequin. tiniest bit of corporate colour on a shirt, i.e. orange and black stripe as long as its subtle will stand you in good stead, but make it very subtle. Maybe orange cufflinks? Get a hair cut too.
Dont worry if you muck something up, just leave the spade at the door and dont try bullshit your way out. Just say sorry i dont know.
And lastly just think about what type of person they want. Hard working, commercially savvy, think on your feet, approachable yet very firm and authoritive and lastly make small talk with the one who makes the final decision, try spark a conversation over coffee or lunch and that will help. Its not what you know at the end of the day
LAstly, their potatoes are out of the same field as waitrose and sainsburys, just not the right size for them guys!