I reckon Beazlebub has summed it up pretty correctly, be it FAA or JAA.
If you can get back into your departure airfield, using
normal operating minima, performance allowing, and taking any additional single engined restrictions into account, then you do not need a take-off alternate. So under JAA rules if you are flying a twin that is CAT3A capable "on one" and you have CAT 3A conditions at your departure airport you do not need a take off alternate, all IMHO of course
.