Leaking boiler?

I have a 3 year old Ferroli Optimax boiler that leaks not from the pipes but from inside the boiler itself and usually starts after the boiler has been off for a few hours (normally through the night!).
This leaking is gradually getting worse and the only way to stop this incessant leaking to to turn the heating on. Any suggestions on whether I'm able to fix it myself before searching for a plumber? Many thanks