Does anyone have experience of suppliers supplying generators to sites to keep them running in case of an emergency?

Any suggestions/recommendations would be gratefully received.

Yep

We have 2 FG Wilson diesel generator, 2 MW each. These provide back up should both busbars fail. They operate on a permanent standby configuration and are tested monthly.

There are numerous suppliers/installers. You need to get one in to determine what your power load/configuration is, what other redundancy systems you have, and what your recovery capacity is.

They are usually only fitted where business continuity is critical, as John C suggested hospitals, ours are needed as we run the national electricity control centre from our premises.