Steel Tube – spec T45

Gooday All

I am checking the availability of steel tube to spec T45 for the repair of the fuselage of an Auster.

Is there a source of tube for the fuselage longerons and also some for the rudder post and spar in the elevators. I do have the repair manual, sorry about the lack of sizes.

If T45 is now unavailable, is there a suitable substitute?

cheers