No need for vapour barrier if it's a well ventilated / draughty loft space, which it looks like it probably is. Just don't seal up the loft.
Judging by the lathe & plaster of the ceiling below, it's a well-ventilated draughty house. vapour barriers anywhere are likely to cause more problems than they solve. Even that sarking had better be breathable.
@NotThamesWater started
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No need for vapour barrier if it's a well ventilated / draughty loft space, which it looks like it probably is. Just don't seal up the loft.