carradice ponchos are good kit (all carradice products are good kit, ive used their super C panniers for years and they keep the rain out easily as well as ortlieb ones) One thing to check before you buy the poncho tho. I dont think it opens out to a flat square, but forms a sort of tent over the cyclist, in the riding position, i think theyre adjustable with velcro straps, but they really are designed for cycling and to avoid being too flappy when doing so. So be worth looking at one before you buy. Theyre also a very thin lightweight wax cotton, which is good, but important to know
If you look at the pic in this link, ( although its not the carradice one,) it kinda shows you the shape of a cycle cape, and the carradice is much the same as that, rather than a flat square/oblong poncho
http://www.bicycleapparel.com/raincapeart.html