Britony System 60 Boiler water pressure?

After bleeding one of my radiators I noticed that the radiators went cold and, upon inspection, I noticed the pressure gauge on the boiler was reading zero. I'm guessing that it's just lost pressure but I can't work out how to re-pressurise the system. The manual doesn't help. Am I right to assume it's a simple re-pressure problem? The boiler has been noisy for a while now so the problem could be more than just water pressure. There must be a valve to re-pressure but I'm damned if I can find it...can anyone help?