Good, thanks for that, as I thought.
Last year I used vetch where I was going to put in late crops like autumnal brassicas so that it had time in the spring to do its thing. It worked really well. There were very few weeds, easy enough to clear, the soil was loose and aerated, and once I had cut at the level of the soil none of it grew back.
But where the soil had been left uncovered or mulched or mucked it did become quite compacted over winter and had to be worked harder with a hoe in the spring to take the new crops – so the mustard sounds like a good option for these areas of the beds.
Cheers