How do I increase the pressure in my Worcester 24i junior combi boiler?

The radiators supplied by my Worcester 24i junior combi boiler were cool at the top, so I proceeded to bleed them. I noticed however that the pressure in my boiler dropped below 1 bar so I stopped! I now need to know how to top up or increase the pressure before I continue. A local kitchen fitter has had a quick look at it without success. He reckons it may be automatic, and will therefore right itself. Apparently there is no obvious top up valve. Any help/advice would be much appreciated. Thanks!