A1 diesel pressure

[Audigirldad]

New to the Forum, and Audi's so thank you for any advice you can give. My daughter recently bought a 2014 Audi A1 1.6 Diesel with 70k on it. Car went great but now has a problem where it keeps blowing a diesel pipe off the rail. Mechanic says its to do with too much pressure on the return. He's confirmed the injectors are fine. He's confirmed the rail is too. he's replaced a small 'valve' (see pic) that sits between the side of the engine and the coolant bottle ( which he says he's never seen before despite working on lots of VW) . All to no avail. He says he's stumped as to what to do next. All advice greatly appreciated.