Thanx guys appreciate the info, I can save a fair bit by going down in size and mm was just worried that the fence is basically a 60sqm sail and wind can exert a shjt load of pressure, the fence is a planning condition for perpetuity and so want it done once and outlasting me with minimum upkeep.
I was going to sink the post down 1m into concrete and if the bottom end was capped with slightly larger plate and the top was capped off would they still need breather holes?
Will have them either powder coated or hammerite/equivalent painted.
So with the advice 80 x 40 x 2.5mm would be ok ? or 3mm or 3.2mm