Exterior boiler water leak

[llew]

2012 build house with an external Worcester Bosch Greenstar Camray oil boiler. Two floors of Uponor wet underfloor heating, with boiler installed externally and level with the upper floor, with gravity-fed oil. Today the underfloor heating manifold pumps started to make a racket as did the boiler pump. Checked the pressure at the boiler to find it was almost non-existent. So I attempted to repressurize only to find the pressure relief overflow pipe at the boiler let it all out again within minutes. I've checked the pressure at the expansion vessel (new last year) and it shows approx 2bar when the water is drained. What could be the problem? Is it simply the 3 bar pressure relief valve on the boiler itself? There appears to be no expansion vessel on the boiler.

 

[SJB060685]

Boiler external and level with upper floor? Anyway fuel supply not an issue either way.
Has the system been fine up until now with just that expansion vessel? Where is the vessel situated and does it have a flexi hose connecting it to system? You could have a blocked or kinked flexi hose.
When you say the boiler PRV released the water in minutes was the boiler running at this time?