While at Telford model show, I saw some really thin wire and tube, including a bag with very short lengths of tube for use as turnbuckles. You slide the wire into the portion of tube and glue this in place.
I was not certain if I could get a 0.2 mm wire into a 4.0 mm OUTSIDE diameter tube, but they offered me a sample of one wire and one tube, which I took.
I have tried this, and the wire actually went in first time. I did it again, twice, to be sure . I am one happy bunny
The product is from Albion alloys, and Hannants carry the wire and tube, but not the bits.
The Albion Alloys website had nothing definite, but a google search found a sub-website at modelskills.co.uk where you can get the little bits, but the wire and long tube is not yet up (!!!).
I therefore have an order in at Hannants, and another at modelskills. I shall rig my current project with it.
Modelskills also stock that range of specialist WWI paints, and I ordered some of those as well.

Clive