Dripping overflow+sealed system?

[nosnam]

Forgive the ignorance!
Overflow dripping hot water continuously at a steady flow! Internet+forums etc refer to in F & E tank in loft - this is a sealed system I assume as no tank in loft? System is a wall hung Ferrolli 700? Any suggestions - hopefully nice and simple answers will arrive and bearing in mind that not being the most technically gifted person in the world, if a registered plumber is required then let me know!
Thanks a lot people!

 

[Baxi-Remeha]

if ur loosing pressure could be expansion vessel, if pressure goes up when boiler is off, could be filling loop letting by or been knocked open.

or if combi, could be plate heat ex passing through.........

 

[kevplumb]

change the prv first then watch the system pressure