If you have a Worcester combination or system boiler it will have a pressure gauge giving a reading in Bars. When the boiler is not running and cold the system pressure should be one bar. The pressure is not exact and will vary by a few degrees depending on the temperature of the water in your Worcester boiler and central heating system. If your system has fallen in water pressure it will reach a point when it cuts out and will not function. Pressure loss is not unusual on any boiler with a sealed pressurised system but it should be remedied.
You should have a Worcester charging key which you insert into the charging link assembly beneath the boiler. You rotate the key and charge the Worcester boiler to one bar when cold. Remove the charging key when done. You should note that if your Worcester boiler continues to lose pressure on a regular basis you should contact a qualified heating engineer to investigate why your heating system is losing water. Continued additions of fresh water will lead to boiler damage like scaling or sludging up of the heat exchanger which will cost a great deal of money to repair or replace.