Liz said, "When she [Limpit] was 2, it [a slap] stopped her biting people and trying to get in the washing machine because she was too young to understand the consquences of those actions. Now she's nearly 11 (ye Gods!!) it is not an appropriate punishment because her communication skills mean we should be able to reason out the argument and make ourselves understood."

Excellent example. When a child is too young for verbal instruction to be effective, sometimes "aversion training" may be the only viable way of helping them learn to avoid dangerous situations (since you are unlikely to be able to physically guard them every second).   Sometimes this is done using "natural consequences," such as letting the kid pull the cat's tail and letting the cat deliver its own lesson.    But obviously letting a toddler learn everything that way can be perilous.   

This is why I find it odd that a law would prohibit spanking under the age of 3. Let me repeat that unnecessary and excessive force should not be sanctioned, nor beating permitted, regardless of age.   But if a slap on the wrist, or on the backside, is ever a very useful teaching technique, it's probably in the first 2 or 3 years.   After that, if you can't keep a kid's behavior within reasonable boundaries by reasoning, time outs, modelling, reinforcement of desired behavior, etc., you've got real childrearing problems and using physical punishment is not likely to make things better.