Mr Walsh
This is a very common problem when given location restrictions and a constant need for kale as a component for cover and potential second year feed. Your strip cropping is a good idea and certainly will have helped to keep any previous problems at bay.
Your use of triticale and linseed is a good base to work from and we would often suggest adding gold of pleasure to provide further feed and bolster the cover potential - all these plants are more forgiving of of poor growing or challenging conditions. This will create an annual mix but will provide a break crop opportunity.
Fodder radish is less susceptible to 'kale sickness' and produces lots of seed however it is an annual crop and its inclusion should be at a very low rate otherwise it will swamp the other elements. Caledonian kale is a variety that is club root resistant which may help but I would still suggest a break from brassicas for a couple of years would be a good plan.
We are having great results with reed millet as a cover component. The initial response is 'millet won't grow here' but we have had good results in Northumberland and the Scottish borders in exposed sites so it may be worth looking at. It will not produce seed but its structure will keep the seed bearing plants off the ground and provide staging posts.
Perennial chicory (well documented on this website) is worth considering and could be added at a low level to give you a second year cover if required - this produces seed and a range of farmland bird species have been known to utilise the crop.
Soil testing and possible lime application would be highly recommended to ensure soil conditions are in order - we often find that 'kale sickness' in many situations is used as a blame all for other underlying issues such as poor seedbeds, incorrect sowing time or loss of crop to pests such as flea beetle, slugs and pigeons.
I hope this helps.