Diesel shouldn't expand enough to make a huge difference with the temperatures involved here, especially as I never brim the tank, so there is more than the standard 5% required by UK regulations. The Jerry can thing, only really an issue with petrol where (rather than the thermal expansion of the liquid) the vapour pressure is the issue.
The original breather had a one way valve fitted, so I'm fairly happy it won't be an issue. I can always whip the valve out if needed

Monitor it & if you get any odd running issues just dump the one way valve.

Ratae said:

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The breather is definitely connected to the correct place, and there was a 1 way valve in the original setup. It breathes fine with the setup I've installed.