A few days ago, I dug the steel tube out and placed it in an electrolysis bath to remove the crud and rust. After several hours, it came out looking pretty good. It had some surface pitting, but was still solid. However, the filter mesh on the bottom was split. An Ebay search revealed some 0.2mm hole brass mesh in an A5 sheet for a few pounds. This arrived today and a circle slightly larger than the tube was cut.

The original mesh was trapped between a lip or ring of metal in the bottom of the tube. I couldn't tell for sure as it had been badly corroded. So I cut out the old mesh and filed the bottom of the tube free of the jagged remains. The outside of the tube was cleaned with a rotary wire brush and then tinned with solder. The new mesh was placed over the hole, flux added and solder applied, melting through the mesh to the tinned tube below, fixing it in place. A little dressing of the solder with a file and the job was done.