Lots of possibilities Eleanor but the answer partly depends on which green manure you are growing eg if mustard, a two inch covering of manure will kill it, but clover, rye grass or vetch would need three to four inches as they are persistent perennials.
Advice depends on so many things eg what are you planning to grow? If sowing carrots or planting lettuce in April, I would spread the manure now to give the green matter time to die and for slugs to diminish. If planting leeks and brussels sprouts in June, you could spread the manure in March. And so forth…!
If your soil is light with few slugs, March manuring is good again. But if clay I would do it now to have bare ground over winter and few slugs by spring.
Pete’s system sounds interesting and good value, 25% of ground is not too much to lose if you have a big plot. I suspect that yields are lower than from using manure but Pete may have a comment on that.