Vokera unica HE boiler blowing hot water out?

Hi I was hoping someone could help regarding my boiler, I have trawled the internet with no success.
We have a Vokera boiler which we have had for 3 or 4 years without too many problems, the fan went last year and we have had a few pressure problems which I think were due to rad leaks.

The problem is I fill the boiler up to 1.5 bar pressure making sure all rads are bled and the pressure still at 1.5, I turn the boiler on and it runs for approximately 5 hours before it shuts off, you can then hear clicking as though its starting to fire up but obviously there is no water.
When I went outside yesterday I noticed a damp area where the blow off is, can anyone help as to what it may be? I am going to call an engineer out today or was thinking off going on one of the British Gas insurance schemes whatever is cheapest. Thanks to anyone that can help kind regards Paul

Paul Wilson
November 2011