Hi Ross - I tried both red and white clovers as an over-wintering green manure (sown in august/September), but wasn't very impressed; not much winter cover, and lots of clover weeds in the growing season!
Depends how you want to use it? As short-term cover, they're too slow growing - alfalfa and fenugreek worked better for me. But if you've a bed that is lying fallow for a lot longer, I suspect they'd be excellent.
Not much help :-(
All best - Gavin