Has the garage done a pressure test in the system?
I'm not sure. I'm not sure why they wouldn't have but they said the system is definitely pressurising somehow
This is the 1.8 diesel
As you have changed the important parts, I can not see anything other than headgasket or the replacements have failed.
Or a pump timing issue. But that would cause running issues
@Jw89 started
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Has the garage done a pressure test in the system?