If it replaces the 48 covers downstairs (the article says there's 2 of these structures), then that's 70000/(60*48) = £24/cover/day, or say £6 per diner (2 lunches and 2 dinners, as it'll likely be running at full capacity throughout given the popularity)
Seems achievable, given that most of the running costs of the kitchen payroll etc are already sunk costs