Just an update...I stripped down my camera this morning as I wanted to remove all the excess baggage (mirror, viewfinder, flash) and I haven't refitted the cold finger yet.
Luckily, after putting it all back together it actually still works
and the image is good i.e. without vertical line pattern. Needless to say it must have been something I did
I'm rethinking the cold finger for the 1100D now. The U-shape is hard to produce without any tools and also hard to fit, with all the protrusions from the screws around the sensor frame.
On closer observation, it should be possible to slide in the copper from the opposite (right, if viewing from the rear) as it appears unobstructed. I'd ignored this before as I wanted to keep the copper slither as short as possible. It must pass by the main circuit board but that already has some shielding on. With a little trimming of the shielding around the sensor circuit board, it should be able to slide in there with only the main PCB removed., whereas the U-shaped required the sensor assembly to be removed first.