Phil L wrote:

Why not use normal breathable roofing felt? - it's cheap and can be
stapled to the joists just the same and will also cut down the amount of
condensation reaching the tin sheeting, with the added bonus of allowing
the old timbers to breathe.

Yes this is what I was asking, would it be better to staple it in between
the rafters or staple it across the rafters horizontally, as one would fix
slater's felt on the top side of the rafters.

I have used the proper Tyvek, under profiled plastic coated steel sheet, on
rooves where we have replaced a certain fibre reinforced corrugated
concrete sheet, and this seems to be working fine but this is on top of the
rafters and in loose but direct contact with the steel sheet.

AJH