Worcester 28Si boiler is leaking when heating is turned on?

Whilst the heating was on, the pressure was really low (0) (has been for a while, so we keep letting more water in to get to about 1.5) so I turned the valve to let more water in -the bar didnt really raise but to my horror water started leaking out from inside the boiler. We turned the heating back off and after a few minutes the leak/s stopped. Tried to turn it on again and still got leaks. We still get hot water with no leaks (no pressure though). We took the front casing off to try and see where exactly the leak is coming from, and it appears to be high up inside the top drum part at the top and back of the boiler.
Can anyone give us any advice?
Thanks.