Sounds like your toner cartridge is going out. Usually you can take it out and shake it a little bit and put it back in and it will turn on correctly. That is the quick fix. The long fix is to replace the toner cartridge. Also make sure your printer's firmware has been updated. But your toner cartridge needs to be replaced.
sorry hit enter too soon! Thanks for getting back to me. It actually turned out to be the toner waste that needed emptying - which we'd tried already, but hadn't done it sufficiently enough. The toner was almost new, so wasn't that, but you're commetns helped us to eliminate what it was/wasn't, so thanks.