I count approximately 38 coils on my old Lanchester 50 round Magazine spring. My new RTG Sten replacement springs have 32 coils, for 32 round. Surely someone could make some with a few extra coils added? Also my custom welded UZI 40 round mag has one small coupler to link two springs together, and it does a fine job. I've had no issues with feeding from this thing. Coupler is a Roll Pin I believe, probably a common size - 2.5mm Outer Diameter / 15.77mm Length
So, I'm saying I believe someone can make new Lanchester Mag Springs, or possibly cut a small section from one Sten mag Spring and couple it to another longer section. Just my opinion - not tested this on a Sten/Lanchester setup yet