The original intention was always to build the luggage from aluminium.
Advantages: Lightweight, corrosion free, available in angle form in the right dimensions.
Disadvantages: Hard to weld.
The disadvantages in this case have outweighed the advantages; after weeks of practicing oxy-acetylene welding aluminium I've come to the conclusion that when each piece of luggage requires 21 distinct welds I won't be able to keep every single weld consistently to the standard I'm looking for.
I've made the decision to make the structure of the panniers in steel, but will still use aluminium sheet for the panels. This is both a setback in terms of time and money spent on the original aluminium angle that I purchases and had cut and bent to size while I continued to practice the welding, however I'm far more confident in being able to weld the steel to the standard I'm looking for. Theoretically the steel should work out as heavier, except that being unfamiliar with aluminium I was using thicker stuff than was probably required to err on the side of safety and so there probably won't be much difference in it.
Working with the steel has already proved much easier than the aluminium, pieces are now all cut and bent to size using my templates, and I'm now making up some jigs to hold the steel square while it's welded.