loss of pressure to my worcester boiler?

I came home last night to find there was no hot water. Looked at my boiler and the pressure guage was under the red bar, believe the red bar is where the guage should be ?

Re filled the boiler up to about 2, on the pressure gauge and it works fine for about five minutes. I have water running from the overflow pipe outside and the pressure is then lost completely down to zero and the boiler fails to work.

I have a worcester bioler 28CDI ????

Only around 4 years old

Any suggestions ??

Becki
October 2008