Shall we bother ......

Don't bother

I am not using brassica cover crops because of the clash with rape just plant a few oats add some phacelia if you can get it cheap enough

CC need drilling early.
If you have weed problems, a CC will make it worse not better

I'd stick to glyphosate doses if it were me.

If you time for cover crops, you haven't got enough sheep!

What are you looking to achieve? Carbon building, nutrient scavenging, soil protection, soil structure improvement etc etc etc? If none of the above, don't bother. If any of the above, depends. Useful eh!

Tricky one. Will be interested to see if the linseed offers any weed suppression, and whether it survives too long after any hard frosts. Don't know enough, non-anecdotal, info about mustard and it's issues with OSR following the spring Barley, (Well aware of Brassicas, but not specifically mustard) but for grass suppression I'd suspect that you'll have little ground cover with only linseed.
Interestingly (probably only to me), we used Winter Barley for similar reasons, to overwinter the soil before Spring Linseed, this year. It was an 18ha field with mapped strips of individual seeds planted in the autumn, in order to establish the best/any issues etc, relating to specific species. Winter Oats, Winter Barley, Winter Beans, winter Wheat, and off and on, Radish/Phacelia/anything else that we had kicking about from wild bird mixes of previous years. Looking forward to harvesting the linseed. Following Winter Barley really struggled to get going, winter Oats was the second poorest looking, followed by the mix, then beans, and the best was Winter Wheat. Have another funny looking field on a Nat Trust property that provokes a lot of questions from the public, due to the linseed being fit to harvest where the beans were, yet still flowering where the oats were. Drilled in 6m bouts....!

Would love to hear how the suppression goes, and also how the spring Barley establishes and yields.

Good luck,

Tim