Cylinder manufacturers are often very optimistic about their cylinders' recovery rate. Sure, to heat 900kg of water from 10*C to 60*C will need approx 180,000 KJ. To do this in 30 minutes (1800 seconds) takes a rate of 100 kJ/s or 100kW. I would say that most cylinder coils in pressurised cylinders are no bigger than 10kW capacity, and that is when cold. As the water heats up the coil power is reduced, maybe to as low as 3kW as domestic water approaches 60*C and deltaT is more like 15K. This means you may only need a 30 or 40kW boiler dedicated to hot water generation.
Please note the correct use of lower case k for kilo, and upper case for Kelvin
However, the good part is that acceptable hot water may be as low as 45*C, so the customer will assume the water has recovered much sooner than the 30 minutes design time.
Consult a competant person with design experience of this type of project, as has been said before here, experience is that which you receive just after you need it.
Also, with constant (assumed), and large hot water requirements, have you considered supplementing the gas boiler with solar panels? Presumably your occupancy levels peak in the summer months when solar generation is at its peak. Be seen by your customers as green too, whilst saving money (and saving a little gas for my kids)
MM