Be Excellent To Each Other

Are after-employment 'can't work for similar company within six months of termination' clauses in contracts legal?

Rarely, although sometimes. Basically only if they employer can prove you have trade secrets in your head.

Aren't employers meant to put you on 'garden leave' if you decide to continue your career working for a competitor? Ie, they continue paying your wages for a month or two to prevent you from heading straight off to your new employer with a head full of trade secrets.

Sometimes, maybe. Generally they're unenforceable, especially if their effect is to make it difficult for you to find work (I worked at a place that had a non-compete clause that said I wasn't allowed to work for a competitor for 6 months after I left - as every other law firm in the country is a competitor, that wasn't going to be enforceable).

99% of the time you can ignore them, basically. The exceptions will be things like where you work on highly sensitive trade secrets and your employer wants to prevent you from working at their (specific, named) main competitor for 6 months or something. And even then only possibly.

As above unless you work for MI6 they are fundamentally unenforceable. I left the consultancy I worked for and a month later started working for my former client...bollocks to the employers.

They whinged and bitched about my employment contract but when I pointed out they were perfectly free to take me to court they pissed off.