Not sure if the longer extension is the problem, but I've seen quite a few sails, over the years, that had exactly this problem.
The combined bend of the mast and the extension is way too stiff, and if the top of the extension is anywhere near the front of the bottom cam, you get rotation issues.
Do you have an shorter extension (like a 26 cm) that you can try?
That might just take care of the poor rotation of the bottom batten.
Also, it's never really a good idea to put too much extension up inside your mast as it does affect (marginally in some cases, alot in others) the overall mast bend curve.
I'm looking at an older NP XT26 (Matrix 2000) extension at the moment, and if I set the ring at 20 cm, I measure 19 cm to the top of the extension.
From the top setting on this extension (26 cm) there is still 13 cm (non adjustable) to the top of the extension.
If you have a 42 cm extension, and it has the same 13 cm of non-adjustable area above the top setting, you would have 13 cm +22 cm (42 cm - 20 cm= 22 cm) or 35 cm
up inside your mast.
Does the bottom batten on the Severne Raceboard sail come anywhere close to 35 cm from the base?
If so, I think you would be alot better off (rotation of the lowest cam) and also you would avoid "point loading" the mast at the top of the extension by using a shorter 26 cm extension.
Mast won't bend at all in the area where it ovelaps the extension, so the mast will want to bend alot more right as it comes past the end of the extension, thus causing significant "point loading" in that area. Add the cam on top of it, pushing in the opposite direction, and you have a good recipe for a broken mast.
Hope this helps,